Calibration FAQs

Every spectrometer we ship comes with a sheet listing its wavelength calibration coefficients (four floating-point values that can be used to derive the wavelength associated with each pixel on the detector), as well as a summary of the spectral lines (from mercury, argon, xenon, neon and other calibrated line sources) used to generate those coefficients.

This calibration sheet contains only a brief overview of the data collected during our automated calibration and alignment process. For example, the pixel values reported on the sheet are “major peak locations” (nearest full-pixel). These pixel values are not sufficient to regenerate and validate the “Predicted λ” column of the calibration sheet.

Our calibration software looks at the surrounding peak characteristics to predict and interpolate a more precise “sub-pixel peak” location. It is this sub-pixel (fractional pixel) that is fed into the linear regression that generates our wavelength coefficients. Without having these fractional peak locations, you will not be able to manually re-generate the “Predicted λ” values to expected levels of precision. (You will arrive at a very close value, well within our accuracy specification, but it may not match the printed value to three decimal places.)

Below is a breakdown of each section on the wavelength calibration data sheet.

1. Header Information

At the top of the sheet, you’ll find the general details about your spectrometer:

1.1 Order Num ber and Serial Number: Identifies your specific unit and purchase.

1.2 Model and Configuration Details:

• Model: The spectrometer model number (e.g., SR-4UVV300-10) provides details about the configuration:
• The first part identifies the spectrometer family.
• The next part specifies the general wavelength range, in this case UV and visible.
• The third part indicates the blaze wavelength of the grating.
• The final number represents the slit width in microns that the spectrometer was ordered with.
• This pattern applies to standard predefined parts and may not follow for custom configurations.
• Grating: Describes the grating used, including the number of lines per millimeter and blaze wavelength (e.g., 600 Lines, Blazed at 300 nm).
• Bandwidth: The wavelength range covered by your device (e.g., 188-906 nm).
• Options: Lists additional components or configurations such as detectors, lenses, slits, or mirrors (e.g., DET4-200-850 Detector, L4-C Lens). This section is primarily for internal use by technical support and sales support.

2. Wavelength Calibration Data Table

The table provides calibration information essential for ensuring accurate wavelength measurements.

2.1 Columns Explanation:

• Pixel #: The detector pixel corresponding to a specific wavelength.
• Predicted λ: The theoretical wavelength value at a given pixel.
• Δλ: The difference (Δ) between the predicted wavelength and the calibration fit. These values represent residuals from the fit and are not a guarantee of long-term accuracy.

2.2 Interpretation:

The calibration process uses a large number of atomic emission sources, each providing a large number of peaks for precise calibration. Small Δλ values indicate a precise calibration fit. Larger deviations may suggest issues or need for recalibration. The table highlights specific calibration peaks across the spectrometer’s range.

3. Calibration Coefficients

These coefficients describe the mathematical model used to convert pixel numbers to wavelengths:

3.1 First, Second, and Third Coefficients: Polynomial coefficients used in the spectrometer’s wavelength calibration equation.

3.2 Intercept: The constant term in the calibration equation.

3.3 Regression Fit: Indicates how well the calibration model fits the experimental data. A value near 1 (e.g., 0.9999995828) demonstrates a very accurate fit.

4. Test Spectrum Plot

This section includes a spectrum graph showing the performance of the spectrometer across its range. The x-axis represents the pixel numbers, and the y-axis represents signal intensity. This plot is a snapshot captured during calibration and is not intended for detailed diagnostics. It provides only high-level information about the system’s overall performance.

5. Stray Light Measurements

Stray light refers to light from one wavelength ending up at a spot designated for another wavelength. These measurements are made using various filters:

5.1 Filters and Results:

• Holmium Oxide (444 nm) and other dye-based filters: Provide specific wavelength measurements to test for stray light at critical wavelengths.
• OG550, RG850, FG3 Filters: Assess stray light rejection at different wavelengths. Values are in absorbance units (AU), where higher values indicate better stray light suppression.

Qualifier: These stray light measurements are primarily for manufacturing processes and long-term performance tracking. They are not designed to be directly interpreted or relied upon by the customer for operational purposes.

6. Calibration Date and Technician

The date of calibration and the technician’s name (e.g., 07-November-2022, Luis Delgado) confirm the calibration’s currency and traceability. Regular recalibration ensures sustained accuracy.

We offer a Quick Start Guide. Download here. 

Absolute Irradiance Calibration uses a lamp of known output (measured in microwatts per square centimeter per nanometer) to calibrate the spectrometer’s response at every pixel. This correction adjusts both the shape and magnitude of the spectrum, producing results in terms of microwatts per square centimeter per nanometer (µW/cm²/nm). When an integrating sphere is used, the output is radiant flux in units of microwatts per nanometer (µW/nm).

Although commonly referred to as “Absolute Irradiance Calibration,” a more accurate term would be “Spectral Irradiance Calibration” or “Spectral Irradiance Responsivity Calibration,” as the calibration process specifically adjusts the device for accurate spectral irradiance measurements.

For more on the Instrument Response Function (IRF), which this calibration compensates for, refer to the Ocean Optics Glossary.

No, we cannot offer a portable standard or service for wavelengths below 200 nm. While some of our spectrometers can detect light below 200 nm when used with an argon purge, no national labs produce portable standards in this range. Calibration services for these wavelengths are only available at specialized facilities, like those at NIST, which use synchrotron-based sources in controlled environments. This process requires bringing your device directly to the facility for calibration, which, as expected, is also extremely costly.

Ocean Optics uses calibration light sources that can be traced to a standard light source characterized at the National Institute of Standards (NIST). This does not mean necessarily that a specific model light source used for calibration was measured at NIST. Light sources change their output over time as the bulb degrades.

We see typically 0.1% per hour at 350 nm and 0.02% at 900 nm with a roughly linear fit for wavelengths in between these two percentage errors.  The customer must decide how much additional error and therefore how much time triggers a recalibration.  50 hours is a typical number for many applications.

Common practice is to keep a NIST traceable light source as a gold standard and use it to calibrate other light sources. Each degree of separation from the NIST-calibrated light source introduces some uncertainty, yielding a total estimated uncertainty of within 10% for most Ocean Optics calibration light sources (a value that is typical for the industry). Repeatability of measurements made with a calibrated system will be much higher, typically within 2%.

You have your equipment and are ready to get started. Visit Measuring Lamp Output Power page to follow the steps to measure successfully your lamp or light source output power.

We offer gas-discharge emission sources for spectrometer wavelength calibration that cover wavelengths ranging from ~250-2500 nm. Refer to the table below to determine the sources best suited for your required wavelength range. By using more emission lines, you can correct more effectively for drift and other phenomena inherent to all spectrometers.

• HG-2: Mercury-Argon (253-1700 nm)
• KR-2: Krypton (427-893 nm)
• NE-2: Neon (540-754 nm)
• AR-2: Argon (696-1704 nm)
• XE-2: Xenon (916-1984 nm)

Yes. We can calibrate for absolute irradiance measurements a spectrometer setup with direct-attach cosine corrector. However, disconnecting the cosine corrector after it’s been radiometrically calibrated will invalidate the calibration. Even if the cosine corrector is carefully reconnected, the exact light coupling won’t be replicated. Any changes to the optics or connections require a full system recalibration.

No! A calibration light source cannot be used as a reference for relative irradiance. The presence of a diffuser in the output path changes the spectrum so that it is not a blackbody source.

No! An absolute irradiance calibration requires that the spectrometer and attached optics be calibrated as a system, since that is the only way to characterize precisely the combined effect of all optical components and optical interfaces.Any change to the spectrometer configuration, fiber, or sensing optics will change the amount of light hitting each pixel, which invalidates the calibration. Even disconnecting the fiber from the spectrometer and reattaching it can change the response of the spectrometer and affect the calibration. In fact, to prevent this, we seal the fiber connections of systems calibrated on-site at Ocean Optics prior to calibration to help prevent accidental disconnection.

The high-reflectivity STAN-SSH-NIST reflectance standard is calibrated to a NIST standard with ~1% accuracy from 250-2400 nm.