Contrast light transmission measurements

The circuit described in this article can perform comparative measurements of light transmission and can be used for optical loss measurements. Losses include reverse and absorption.

Figure 1 shows the optical transmission measurement circuit. The ICL7106 reference voltage is set to 100mV. When there is no board under test, select R1 to display 100. The plate under test is then placed and the display gives a reading of the percentage of light transmission through the plate.

Figure 2 shows the optical loss measurement circuit. This circuit is the same as Figure 1. The difference is that -100mV Vref is added to the non-inverting input of the op amp. The display reads zero when there is no plate being tested. A completely opaque plate has a reading of 100. So the display reads out the light loss (reflection plus absorption).

Op amp IC1 should choose an op amp with very small input bias current such as CA3130 or MAX406. The interior of the opaque box should be finished in opaque black. The photodiode type should be suitable for the light source, for daylight choose BPW21. It has a response similar to that of an eye.