Measurements of WB reflectance targets.

[ulf]

Steve (Camium) kindly sent the material pieces on the photo in post #21, a ca 1mm thick filter mat and a thin round filter membrane, 55mm in diameter.

Please see the measurement results below.

 

Measurement preparations and verification:

 

During the setup of this second session I realised that my Premium-grade reflection probe, QR400-7-SR-BX, gave a slightly different illumination area from the two different lamps in my DH-2000-BAL light-source.

This might explain the bumps in the graphs in post #1 and other measurement difficulties I have had.

 

This time, when using the deuterium lamp only, I have seen much more convincing measurement results, but only between 225nm and 425nm.

 

I have made an alignment gig for the probe to place the tip at ca 15mm from the surface, well aligned at an angle of 90° to the surface.

This give an illumination spot from the deuterium lamp of ca 8mm.

 

After a suitably long warm-up time I calibrated the setup against my 4mm thick matted pure PTFE sheet.

Directly after the calibration I made a verification measurement after removing and reapplying the probe to the reference sheet.

I made the same verification measurement after the full measurement session.

The same was done checking the black offset calibration of the spectrometer.

 

Red graphs from before the session and green graphs from after the session:

 

The measurement results:

 

The 1mm thick filter material show a rather even reflectance:

Red: PTFE Reference - , Green: - the 1mm thick material, when measured, placed either against black or PTFE as background.

 

The bright green curve show that the material transmit some light that is bounced back through the material.

 

The thin 55mm filter membrane is a bit transparent.

Only a measurement against a black background, to avoid added responses from the background, is meaningful.

Here the black background is a dry surface painted with Semple Black, causing minimal interference.

 

This material on black background agree even better to the virgin PTFE used as a reference.

 

Conclusion:

 

The spectral range we use these materials for, for WB is limited by the UV-pass filter used, the camera-sensor sensitivity and the spectrum of the available light.

 

A typical image taken in sun-light, with a filter like a Baader U, is dominated almost completely by light in the range of 360nm - 390nm, mainly due to the decreasing sensitivity of camera sensors with bayer dyes at shorter wavelengths.

In this range, all materials above have a reflectivity that is changing less than 1%, making them quite good for WB.

[Andrea B.]

It has been very enjoyable and useful to read about making and measuring reflectance targets. Thanks from UVP for everyone's efforts on this. :D