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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?
What light source can I use for a relative irradiance calibration?
What sampling optics can I use for relative irradiance?
What spectrometer can I use for detection in relative irradiance mode?
What is absolute irradiance?
What light source can I use for an absolute irradiance calibration?
What sampling optics can I calibrate for absolute irradiance?
What sampling optics can be calibrated for absolute irradiance at Ocean Optics?
What is the collection area of my sampling optics?
What spectrometer can I use for detection in absolute irradiance mode?
How do I make radiometric, spectroradiometric, or photopic measurements?
Tips for the best possible calibration and measurements
What is upwelling/downwelling? What system do I need?
What system can I use for LED analysis?
What system is best for solar irradiance measurements?

 

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.

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
Torus

Torus

High Throughput Compact Spectrometer for Color Measurements and Light Metrology
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