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Photodiode

# How to measure light absorption percentage using a photodiode

## How to measure light absorption percentage using a photodiode

let’s trying to find the absorption of some sort of glass filter plate.

connect your photodiode (pd) to a series resistor and polarize the combination in the opposite direction to about 5 v. a resistance of 10 kilo-ohms should produce a voltage within a reasonable range. you can experiment with value.

the response you want to measure is the voltage across the resistor. the light intensity reaching the pd will be proportional to this voltage, unless the pd is saturated.

To make sure the pd does not saturate while you are measuring, move the light source you use closer to the pd until the voltage does not change, then set a small distance.

A bright led will probably work as a light source but must be close. the sun is not a good choice for the light source.

Once you have this initial distance, you can use the initial reading as a reference. call this tension vo before calculating the absorption, you want to know how much light is reflected by the transparent glass (of the same material as your filter). Place the transparent plate between the led and the pd and measure the voltage v1. 1 – v1 / vo is the reflection. (v1 / vo is the transmitted light fraction)

now remove the transparent plate and replace it with the filter. if the measured voltage is v2, the absorption coefficient of your filter material is 1 – v2 / v1.

The right filters will have an anti-glare coating. If this is the case, your clear glass was probably not a good reference for the measurement. if you trust the anti-reflective coating, the absorption factor of your filter is 1 – v2 / vol.

In both cases, you have found an upper and lower limit on the absorption of the material in the filter.

Remember to move neither the PD nor the light source once you have configured. Observe the voltage across the resistor over time to make sure the light source is stable. the intensity of the light striking the pd is inversely proportional to the square of the distance to the light source; a small change in distance will make a big difference in your reading. you must tighten the light source and the PD firmly in place.