ISFH CalTeC: Spectroradiometers

We use a mercury lamp and a 250 W or 1000 W tungsten-halogen lamp as reference light sources. The test includes the following steps:

1. Verification of wavelength calibration

• Most spectrometers are factory-calibrated for wavelength, where each pixel is assigned a specific wavelength.
• This mapping may shift over time, especially due to vibration or mechanical stress.
• We verify this calibration using a mercury vapor lamp, which emits narrow spectral lines at precisely known wavelengths.
• If inaccuracies are detected, we attempt to readjust the calibration.

2. Measurement of raw signal

• Your spectrometer measures the spectral irradiance of the reference lamp in digital units (counts).
• The reference spectrum is known across the 300 nm to 2500 nm range.

3. Calculation of the transfer function

• The measured counts are compared to the known irradiance (in W/m²·nm) of the reference source.
• This defines a transfer function that converts your spectrometer’s readings into physical units.
  → From "counts" to W/(m²·nm)

4. Calibration validation

• The reference source is measured again to validate the calibration.
• Results are documented in a DAkkS-compliant calibration certificate.

Measuring spectral irradiance requires a spectroradiometer—an instrument calibrated for both wavelength and absolute spectral irradiance.
Most spectrometers come with only wavelength calibration, using narrow spectral lines of known wavelengths and interpolating the values in between.

However, this does not account for the optical path, including mirrors, filters, gratings, or detector response, which all vary with wavelength. As a result, the measured spectrum does not represent the true spectral irradiance.

To correct this, a transfer function is needed to convert the detector’s arbitrary intensity scale into calibrated irradiance values.

ISFH CalTeC is DAkkS-accredited for the testing of spectroradiometers (registration number D-PL-18657-01-00).

Our accredited testing focuses on:

• Accurate determination of spectral irradiance
• Wavelength accuracy in the range 250 nm to 1700 nm

These tests are performed in accordance with the international recommendations of the CIE, specifically CIE250:2022 “Spectroradiometric Measurement of Optical Radiation Sources.”

Calibrating spectrometers for measurements of the spectral irradiance caused by solar radiation
Paper: Metrologia 57 (6), 065027, (2020) – open access

Implementation and uncertainty evaluation of spectral stray light correction by Zong’s method
Applied Optics 58 (36), 9998, (2019) – open access