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Irradiance

absolute relative irradiance ocean optics spectroscopyThe human eye perceives the intensity of light on a logarithmic scale, which means that light appearing twice as bright is actually ten times as bright. Add in the fact that the eye responds more strongly to green light than other wavelengths, and it becomes obvious that instrumentation is needed for even the most basic comparisons of light intensity.

 

Spectroradiometers fill that need, providing detailed irradiance data. Irradiance is the amount of energy at each wavelength emitted from a radiant sample. From that data, more specific values can be calculated, including moles of photons, PAR, and photopic values like lumens, lux, and candela.

 

In reality, however, few users need to know the absolute amount of light. Relative irradiance measurements are an excellent alternative when only the shape of the emissive sample is needed.

 

Regardless of the emissive source being measured, intensity calibration is required. This applies even if the goal is to simply make color measurements of an emissive source like an LED, computer display, or light source. If the sample under study is emissive, you must begin your measurements by calibrating.

 

We’d like to guide you through the options for irradiance measurement, and take you behind the scenes of the magic act we call irradiance to demystify it. Armed with a little understanding, you will be able to select the best system for your application. The handy wizards in our software will take care of the rest. Presto!

 

Advantages

  • Quantitative: Far more accurate than human eye, with detailed spectral information
  • Flexible: Irradiance data acquired can be used to calculate other power parameters or color

Applications

Other Common Applications

  • Solar research: studying greenhouse gases in the atmosphere and ozone depletion, investigating the effect of solar radiation on ecological systems and crops, evaluating the effect of UV sunlight on skin and eyes
  • Biology: measurements of upwelling & downwelling for photosynthesis research, hydroponics
  • Industry: monitoring plasmas, analysis and binning of LEDs, studies of photo degradation, characterization of tanning beds, street lighting, advertising signage, and UV curing lamps

Technical Note

What Is Scope Mode, and Why Can’t I Use It for Irradiance?

Scope mode data shows the raw number of counts for each pixel in the array without any processing or correction for spectrometer sensitivity. This is important to remember, because each spectrometer has a different response function that comes from a combination of its individual elements and alignment. That can make scope mode misleading, showing a peak in the right general location, but with a distorted shape and/or center wavelength.

Scope mode spectrum vs true spectrum:

Figure 1: Scope mode spectrum of a tungsten light source

Figure 1: Scope mode spectrum of a tungsten light source

Figure 2: True spectrum of a tungsten light source

Figure 2: True spectrum of a tungsten light source

Instrument Response Function

Each spectrometer has its own unique wavelength dependent response, called the instrument response function (IRF). Every optic encountered after the point of light collection contributes to the IRF, including but not limited to fibers, lenses, grating, mirrors, filters, and detector. The only way to easily and accurately correct for the many contributing optics is to calibrate the spectrometer against a known standard.

Grating reflectivity x Detector efficiency x Other optics = Instrument response function

Ocean Optics offers two options for irradiance measurements. Relative irradiance mode uses a calibration against a blackbody light source of known color temperature. It results in a spectrum with the correct spectral shape, scaled from 0 to 1 in arbitrary units. Absolute irradiance mode requires calibration against a NIST-traceable light source, but offers data in absolute units of power or energy. In either case, it is important to always calibrate using any optics and/or fibers that will be in the optical path during the final measurement.

Featured Products for Irradiance – LED Analysis:

Irradiance - LED Analysis Setup

Torus Torus spectrometer (360-825 nm) has low stray light and high thermal stability. Your choice of optical bench accessories very much depends on the LED wavelengths you are measuring.
FOIS-1 Integrating sphere that collects light from emission sources such as LEDs
LED-PS Power supply powers the LED, displays the LED drive current and holds the LED in place
QP400-2-VIS-NIR Premium-grade patch cord directs light collected at integrating sphere to the spectrometer
HL-3-INT-CAL Radiometrically calibrated light source designed for use with an integrating sphere
OceanView Spectroscopy software

Featured Products for Irradiance – Upwelling/Downwelling:

Irradiance - Upwelling-Downwelling Setup

HR4000 High resolution spectrometer. We recommend configuring the spectrometer with a grating for extended range (200-1050 nm), a 50 µm slit as the entrance aperture and a detector with variable longpass filter and quartz window.
DH2000-CAL UV-NIR light source used to calibrate the absolute spectral response of a radiometric system
HL-3 series Calibrated Does not have quite the same range of response as the DH2000-CAL, but may be a suitable alternative for some applications

What is relative irradiance?

A relative irradiance measurement corrects the shape of a measured spectrum without defining the scale. It simple scales it from 0 to 1. Calibration is performed by sampling a blackbody light source of known color temperature. The software then uses the color temperature to calculate the theoretical spectral shape of the source and apply a wavelength-by-wavelength correction to the full spectrum.

Calculating relative irradiance looks a lot like a transmission calculation, with the addition of the blackbody reference shape, B(λ).

Irradiance

 

where:

B(λ) = Theoretical shape of reference (blackbody equation)

S(λ) = Sample spectrum, in counts

D(λ) = Dark spectrum, in counts

R(λ) = Reference light source spectrum, in counts

 

where:

ʎ is the wavelength in meters

T is the temperature of the blackbody in Kelvins

h is Planck’s constant (approximately 6.626 * 10-34 J*s)

k is Boltzmann’s constant (approximately 1.38 * 10-23 J/K)

c is the speed of light (approximately 3 * 108 m/s)

e is the base of the natural logarithm (approximately 2.718)

 

Relative irradiance measurements work well for measuring the center wavelength of an LED or other limited bandwidth light sources. They can be used in combination with a basic power meter for quality control on production lines to inspect light sources or other emissive sources. A relative irradiance spectrum is also a good starting point for color measurements of an emissive sample like a computer display.

A relative irradiance calibration will work for many applications. It is much more flexible than an absolute irradiance calibration, since any type of sampling optics can be used, and recalibration is quick and easy to do after sampling optics are interchanged.

What light source can I use for a relative irradiance calibration?

Any blackbody light source can be used for a relative irradiance calibration. A tungsten halogen light source is a convenient standard, and works well for visible and NIR wavelengths. All of our tungsten halogen light sources have a blackbody emission spectrum, though their color temperatures vary. Be sure to use the correct color temperature, and enter it into the software in degrees Kelvin.

Light source Color temperature
LS-1 3100 K
LS-1-LL 2800 K
HL-2000 2960 K
HL-2000-LL 2800 K
HL-2000-HP 3000 K

What sampling optics can I use for relative irradiance?

Thanks to easy calibration, any sampling optic can be used for a relative irradiance measurement. The choice of optic will depend on the field of view desired, and the goal of the measurement.

Bare fiber: gives a 25° full angle field of view, sampling a spot size with a diameter equal to ½ the distance to the object or plane of measurement. It works well for looking at the light coming from a general area, such as the sky.

Flame loop or other probe: gives the same 25° full angle field of view as a fiber.

Collimating lens: if properly aligned for collimation, it samples a spot size equal to its diameter. Collimation can be checked by routing a light source in reverse through the fiber/lens pair. It works well for sampling light from a specific location at a distance.

Gershun tube kit: allows control over the field of view from 1° to 28° using various apertures and configurations. It works well for sampling light from a specific location at a distance, or with a particular field of view.

Cosine corrector: has a 180° field of view, with a Lambertian response. The amount of light collected will vary as the cosine of the angle of incidence on the cosine corrector. It samples any light incident on the diameter of its white diffusing surface, and specifically in that plane. It works well for looking at how much light is incident on a surface, since ambient lighting or sunlight tends to illuminate surfaces from a wide range of angles.

Integrating sphere: has a 180° field of view at the location and in the plane of its sample port. There is no weighting by angle (unlike a cosine corrector), as all the light that passes through the sample port opening gets sampled by the sphere equally. It works well for looking at how much light per unit area is passing through a specific plane or incident on a surface. It is particularly well suited to measuring the total light output of any source that can be completely enclosed by the sphere, like an LED.

What spectrometer can I use for detection in relative irradiance mode?

Any spectrometer can be used for relative irradiance measurements, provided it has the right sensitivity for the measurement. We offer a wide range of preconfigured spectrometers, or can create a custom spectrometer for your specific wavelength range and application. Contact one of our application sales engineers to discuss your needs.

What is absolute irradiance?

Irradiance is the measurement of radiant flux per unit area hitting or passing through a surface. An absolute irradiance measurement results in a spectrum that is accurate in both shape and magnitude. The y-axis scale becomes scaled in power or flux units like μW/nm or μW/cm2/nm, making it easy to calculate other power or energy values.

Measurements in absolute irradiance mode require a calibration using a source with known power output. A spectrum is measured with the sampling optic (fiber tip, CC-3 cosine corrector, etc.) connected to the calibration light source, and is then compared to the known output power of the calibration light source. Remember to always use the light source calibration file specific to the sampling optic being used, and calibrate just prior to measurement if possible.

The calibration process generates a file with energy response data for each pixel in the CCD, given in μJ/count. Factoring in the surface area of the sampling optic and the integration time allows irradiance measurements in μW/cm2 to be reported (power=energy/time). Calibration is only possible if the absolute power output of the calibration light source is known, so if the light source calibration data is not given in the units μW/cm2/nm, it may not be a light source capable of calibrating for absolute irradiance measurements.

Calculating absolute irradiance takes into account the collection area of the sampling optic, and is corrected using the calibration data for each pixel, CP.

Irradiance , where

Cp = Calibration file, in μJ/count (specific to the sampling optic)

S p = Sample spectrum, in counts

D p = Dark spectrum, in counts

T = Integration time, in seconds

A = Collection area, in cm2 (A=1 for an integrating sphere)

dLP = Wavelength spread (how many nanometers each pixel represents)

What light source can I use for an absolute irradiance calibration?

An absolute irradiance calibration requires a light source with known spectral output power at certain distance, such as the HL-2000-CAL, DH-2000-CAL, HL-3 or DH-3 series calibration light sources. Each version of these light sources is designed to be used with specific sampling optics (e.g. cosine corrector, integrating sphere or fiber) and a separate calibration file is included for each sampling optic. In addition, the calibration range of these light sources can be extended to 2200 nm by including the EXT option.

Light source Sampling optic options Calibration range Accuracy*
HL-3plus-CAL-(EXT) Cosine corrector 350 – 1050 (2400) nm 3%
HL-3plus-INT-CAL-(EXT) Integrating sphere (6+ mm port) 350 – 1050 (2400) nm 7%
DH-3plus-CAL (EXT) Cosine corrector 210 – 1050 (2400) nm 3.7%
DH-3plus-BAL-CAL-(EXT) Cosine corrector 230 – 1050 (2400) nm 4%
* at 900 nm

Once calibrated, a light source will maintain its calibration for a total of 50 hours of use (including warm-up time). This time limit is due to the natural decrease in the intensity of output of a tungsten bulb over time, and is typical for any calibrated light source. After 50 hours of use, the calibration light source must be recalibrated at Ocean Optics to ensure accurate measurements.

What sampling optics can I calibrate for absolute irradiance?

Fewer options are available for sampling optics when making absolute irradiance measurements, but fortunately they address the most common applications and needs. It is critical to check that the calibration source is properly matched to the sampling optic to ensure that the field of view is filled during calibration, and that the correct calibration data is used. For example, power output from a calibration light source will be different when a bare fiber is attached than when a cosine corrector is inserted.

Bare fiber: gives a 25° full angle field of view, sampling a spot size with a diameter equal to ½ the distance to the object or plane of measurement. It works well for looking at the light coming from a general area, such as the sky.

Cosine corrector: has a 180° field of view, with a Lambertian response. The amount of light collected will vary as the cosine of the angle of incidence on the cosine corrector. It samples any light incident on the diameter of its white diffusing surface, and specifically in that plane. It works well for measuring the power incident on a surface, in which case it should be placed flush with that surface for measurement.

Integrating sphere (ISP-50-8-I or ISP-80-8-I): has a 180° field of view at its sample port, and measures all light that passes through its opening. In this configuration, it measures the average radiant flux of a light source at a specific distance, in the plane of the sample port. It is particularly well suited to measuring the total light output of any source that can be completely enclosed by the sphere, like an LED. Similarly, it can measure a light source if its full power can be directed into the port (as in the case of a beam, for example).

What sampling optics can be calibrated for absolute irradiance at Ocean Optics?

Ocean Optics can calibrate on-site with any of the sampling tips described above, as well as with other sampling optics like flame loops, direct-attach cosine correctors, or other integrating spheres. A direct-attach cosine corrector works well for use in the field, where fibers are likely to get damaged, and recalibration is not easy to perform. We can also calibrate a customer-supplied optic provided the collection area is known.

Once in our metrology lab, we will calibrate your system using a NIST-traceable light source for which the irradiance as a function of distance is known. This gives us the flexibility to calibrate a wide range of sampling optics, extended wavelength ranges, and at particularly high and low intensities.

If the calibrated system includes a fiber, care must be taken during use to avoid damage to or stress on the fiber and fiber junctions to maintain the accuracy of the calibration. This is particularly important when an integrating sphere is used for collection, as the integrating sphere is significantly heavier the fiber attached.

Absolute Irradiance Calibration

Absolute Irradiance Calibration

Absolute Irradiance Calibration

Absolute Irradiance Calibration

What is the collection area of my sampling optics?

The collection area of the sampling optic is needed to calibrate a system. The software allows this to be entered as a fiber diameter in µm, an area in cm2, or as an integrating sphere.

 

Cosine Corrector with 180° Field of View

Cosine Corrector with 180° Field of View

Fiber and Cosine Corrector

Fiber and Cosine Corrector

 

  • If a bare fiber is used, enter the fiber’s core diameter.
  • If a CC-3 or CC-3-UV cosine corrector is used, it should be treated as a 3900 µm diameter fiber.
  • If a CC-3-DA cosine corrector is used, it should be treated as a 7140 µm diameter fiber.
  • If an integrating sphere is used to collect light external to it, enter the port diameter in µm.
  • If an integrating sphere is used to enclose the sample and collect all light internally (such as an LED), choose the integrating sphere option; collection area is not applicable in this case.

What spectrometer can I use for detection in absolute irradiance mode?

Any spectrometer can be used for absolute irradiance measurements, provided it has the right sensitivity for the measurement. We offer a wide range of preconfigured spectrometers, or can create a custom spectrometer for your specific wavelength range and application. Contact one of our application sales engineers to discuss your needs.

How do I make radiometric, spectroradiometric, or photopic measurements?

Once an absolute irradiance spectrum has been measured, it can be used to calculate a lot of other power measurements. Radiometric measurements describe total power (integrated over wavelength). Spectroradiometric measurements describe power as a function of wavelength.

Photopic measurements describe power as perceived by the human eye (weighted by the unique response function of the eye and then integrated over 380 – 780 nm). Scotopic measurements are similar, but describe power as perceived by the human eye under very low light level conditions; the eye then accepts a wider cone of light, hitting different receptors and altering the response curve of the eye.

Relative Irradiance

Absolute irradiance data can also be used to calculate the number of photons, moles of photons, dBm (for lasers), and eV. Even photosynthetically active radiation (PAR) can be calculated. Many of the values described above can also be integrated over a user-defined wavelength range using the software. Regardless of the value being calculated, it is always a good idea to save the full irradiance spectrum so that the data can be reanalyzed later, in a different way if needed.

  Radiometric Spectroradiometric Photopic
Flux PowerWatts Power/wavelength intervalWatts Luminous flux (as perceived by human eye **)Lumens
Flux/area IrradianceWatts/m2 Spectral IrradianceWatts/m2/nm IlluminanceLumens/m2 = Lux
Flux/solid angle  * (Radiant) intensityWatts/sr Spectral intensityWatts/sr/nm (Luminous) intensityLumens/sr = candela
Flux/area solid angle RadianceWatts/m2/sr Spectral RadianceWatts/m2/sr/m LuminanceCandela/m2 = nitLumens/m2/sr = nit

* sr = steradian, a unit of solid angle; 4π steradians is a sphere

Tips for the best possible calibration and measurements

  • If possible, calibrate the system on location, just prior to measurement.
  • Allow the calibration light source warm up for at least 20 minutes. Always calibrate with any optics and fibers that will be in the path during the measurement.
  • Use a high number of averages, particularly when performing the calibration scan, to achieve the best results.
  • Always keep a log of calibration light source use, including date of last calibration and subsequent hours of operation (including warm-up). Light sources must be recalibrated after 50 hours of use to maintain accuracy.
  • Validate the absolute irradiance calibration by measuring the calibration light source once and then overlaying it with the light source calibration file. You can then hide the light source calibration using the same menu item.
  • Don’t change the boxcar smoothing value after it is set during calibration.
  • Though integration time and averaging may be changed while making measurements, do remember to store a fresh dark spectrum with those parameters. The software will take care of the rest.

What is upwelling/downwelling? What system do I need?

Upwelling is radiation from the earth’s surface, whether reflected solar light or emitted terrestrial radiation. Downwelling radiation is directed toward the Earth’s surface from the sun or atmosphere. The relationship between the two (albedo) can be used to derive spectral information from vegetation, forest canopies, sea beds and more that aid in ecological and photosynthesis studies.

An extended range spectrometer like the HR4000 with an HC-1 grating and 50µm slit covers a 200 – 1050 nm wavelength range with ~1.5 nm resolution (FWHM), giving enough sensitivity, resolution, and range to properly study this type of radiation. A two-channel Jaz system can be used as an alternative, both for portability and to allow upwelling and downwelling measurements to be captured simultaneously.

The two most popular sampling optics chosen for this application are a cosine corrector and a Gershun tube.  A CC-3-UV cosine corrector gives a full 180° field of view, while a Gershun tube allows a specific field of view to be chosen (in variable increments from 1° to 28°). Either can be used in combination with a calibration source and software to calculate spectral intensity and photopic data in lumens, lux or candela.

What system can I use for LED analysis?

LEDs are used extensively in industry for uses as varied as indicator lights and biomedical devices. Characterizing the output power, wavelength, and spectral shape of LEDs is an important part of their manufacture and binning for end use. An integrating sphere that encloses the LED is the simplest way to capture all emitted light and exclude ambient light from testing.

An FOIS-1 integrating sphere and spectrometer mated with an LED-PS power supply provides both the electronics to consistently drive single, undeployed LEDs under test and measure their output accurately. This integrating sphere can be calibrated using an HL-3 series calibration lamp for integrating spheres, creating a system capable of measuring photopic output in lumens, dominant and peak wavelength, and CIE color coordinates of LEDs.

If the LEDs have already been integrated into a subassembly or system, a spectrometer with a cosine corrector may be more appropriate to measure color and output at a specific distance from the LED. This type of system can even be made fully portable using a Jaz spectrometer with battery module.

What system is best for solar irradiance measurements?

Solar irradiance spectrum with identification of absorption bands of atmospheric elements.

Solar irradiance measurements are critical to a variety of fields of research, including atmospheric studies of greenhouse gases and ozone depletion, the effect of solar radiation on ecological systems and crops, and effects of UV sunlight on skin and eyes. Measurements of upwelling and downwelling radiation are also important in ecological and photosynthesis studies.

Solar radiation spans a wide range of wavelengths, from UV to NIR. Maximum coverage of this range can be achieved using a preconfigured HR spectrometer (200 – 1100 nm) or a JAZ-EL200 (200 – 1025 nm). Of the two, the Jaz offers much greater portability, with extended battery life and on-board processing. A Jaz spectrometer calibrated on-site at Ocean Optics with a direct-attach cosine corrector even carries its calibration data with it on an SD card that fits into the battery module. It is also Ethernet-capable, allowing data to be reported back to a field station, or a laboratory in another country.

 

Irradiance Irradiance Irradiance

Radiometric Calibration Resources

Here are some useful resources should you choose to radiometrically calibrate your spectrometer yourself using one of our traceable light sources.

BUNDLE-HR-PLASMA

BUNDLE-HR-PLASMA

Application-ready System for Plasma Monitoring
Flame Spectrometer

Flame Spectrometer

High Thermal Stability, Interchangeable Slits
STS-VIS-RAD

STS-VIS-RAD

Spectral Irradiance in a Tiny Footprint
JAZ Spectrometer

JAZ Spectrometer

Handheld Spectrometer for UV-Vis Measurements
USB2000+ (Custom)

USB2000+ (Custom)

Custom Configured Spectrometer for Setup Flexibility
STS Developers Kit

STS Developers Kit

Connect, Code, Create with New Sensing Tools
STS-UV

STS-UV

UV Spectral Analysis in a Tiny Footprint
USB4000 (Custom)

USB4000 (Custom)

Custom Configured for Maximum Flexibility
HR4000 (Custom)

HR4000 (Custom)

High Resolution Spectrometer for Maximum Flexibility
STS-VIS

STS-VIS

Vis Spectral Analysis in a Tiny Footprint
HR2000+ (Custom)

HR2000+ (Custom)

Custom High Resolution Spectrometer for Maximum Flexibility
QE Pro-ABS

QE Pro-ABS

High-sensitivity Spectrometer for Absorbance
STS-NIR

STS-NIR

NIR Spectral Analysis in a Tiny Footprint
USB2000+UV-VIS

USB2000+UV-VIS

Application-ready Spectrometer for the UV-VIS
HR2000+CG

HR2000+CG

High Resolution Spectrometer for Biological and Chemical Applications
EMBED Spectrometer

EMBED Spectrometer

Robust, Stable Spectrometer for OEM Applications
USB2000+XR1

USB2000+XR1

Extended Range Spectrometer for UV-NIR applications
HR4000CG-UV-NIR

HR4000CG-UV-NIR

High Resolution Spectrometer for Laser Characterization
HR2000+ES

HR2000+ES

High Resolution Spectrometer with Enhanced Sensitivity
USB2000+UV-VIS-ES

USB2000+UV-VIS-ES

Application-ready Spectrometer for the UV-VIS with Enhanced Sensitivity
USB2000+VIS-NIR

USB2000+VIS-NIR

Application-ready Spectrometer for the Visible and near-IR
USB4000-VIS-NIR

USB4000-VIS-NIR

Application-ready Spectrometer for the Visible and near-IR
USB4000-UV-VIS

USB4000-UV-VIS

Application-ready Spectrometer for the UV-VIS
USB4000-XR1

USB4000-XR1

Extended Range Spectrometer for UV-NIR applications
USB4000-UV-VIS-ES

USB4000-UV-VIS-ES

Application-ready Spectrometer for the UV-VIS with Enhanced Sensitivity
USB4000-XR1-ES

USB4000-XR1-ES

Extended Range Spectrometer for UV-NIR applications with Enhanced Sensitivity
USB2000+VIS-NIR-ES

USB2000+VIS-NIR-ES

Application-ready Spectrometer for the visible and near-IR with Enhanced Sensitivity
USB2000+RAD

USB2000+RAD

Spectroradiometer for Irradiance Measurements
USB4000-VIS-NIR-ES

USB4000-VIS-NIR-ES

Application-ready Spectrometer for the visible and near-IR with Enhanced Sensitivity
USB2000+XR1-ES

USB2000+XR1-ES

Extended Range Spectrometer for UV-NIR applications with Enhanced Sensitivity