On the Use of White Light Source for Imager Spectral Response Measurements

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Aphesa publishes a blog post "Why You Can't Use A White (Broad Spectrum) Light Source For Your Spectral Measurements." Few quotes:

"The R, G, B or the monochrome curve is not the response curve of the sensor but only some information about it.

First of all, it is usually only measured for an incident beam perpendicular to the sensor, or in other words at a zero chief ray angle. But we know that the spectral response varies in intensity and in shape with the angle of incidence.

Secondly, it is a noisy measurement and therefore the precision depends on the number of samples taken. The measurement is noisy because the amount of light after the monochromator is limited as only a tiny fraction of the source's spectrum reaches the sensor. Therefore the signal level is small and its SNR is therefore low.

The shape of the spectrum also depends on the bandwidth of the measuring instrument. As the response spectrum can exhibit oscillations at several scales (see this other publication about spectral response), only a very small bandwidth, i.e. a narrow monochromator lid, can reproduce the actual shape of the response curve, any other approach will only produce a smoothed curve without any detail.

Finally, the wide band light source will change over time and its spectrum will change over temperature, therefore requiring regular calibration.

The light source that we use is based on a femtosecond laser and a supercontinuum.

The femtosecond laser is a solid laser that provided very high power light pulses with a duration in the range of the femtosecond. Lasers are known to provide coherent, repeatable and high intensity light. Our laser is red and the laser has the size of a small table.

The role of the supercontinuum is to turn the monochromatic laser light into a wide spectrum by a collection of non-linear processes. The supercontinuum is a long microstructured optical fiber. The fiber seems red at its beginning and is white in the end as the spectrum broadens along the fiber.

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