How to build a simple Photometer

Joachim Köppen Strasbourg March 2010

A photometer is an instrument to measure the intensity of light. As discussed here one can use several different electronic components to do this. Light dependent resistors (LDR, photoresistor) can be used to measure quantatively the intensity and the colour of visible light, as their spectral sensitivity peaks somewhere in the green region, where also the human eye is most sensitive. They are cheap and readily available at various resistance values, and only need the use of an ohmmeter, e.g. an inexpensive digital multimeter. A few example are shown below, on the left hand side.

Light falling on the device's front surface from any direction will cause its resistance to become smaller. If one wants to measure the light intensity from a certain direction, such as an illumination source rather than the ambient light, it is best to put the photoresistor in a small housing with an opening, or a small tube of opaque material, which lets light come in only from a small angle. Below are shown 5 cm long plastic tubes which permitted to measure the light from an angle of about 5 to 10 degrees, which was needed to measure the blueness of the sky in an experiment carried up by a sounding rocket:
This collimating tube can also be constructed from black cardboard. One best uses a material which is not shiny, because otherwise there will be reflections on the inner surface which would compromise the purpose of the tube to restrict the field of view for the photoresistor. To prevent these reflections, one can use baffles, which are apertures in the tube itself, as shown below.
While it would be a bit complicate to fabricate disks with holes of suitable size and insert them in the tube, we can use plastic film canisters put into each other to form the detectos tube. In each bottom of the canister we cut a round hole, stick each canister into the next one. Two or three canisters make a nice collimating tube giving a well-defined field of view:
Glue or sticky tape holds the canisters together, and a lid holds the photoresistor.

The resistance of the semiconductor material in a photoresistor decreases with the intensity in a non-linear fashion, which is close to a power law:

(I/I0) = (R/R0)^(-gamma)

where R0 is the resistance at the intensity I0. The constant gamma is given in the data sheet of the device, and is usually around 0.6 ... 0.8.

To make measurements, the device thus needs to be calibrated. But even without this, one may compare measurements by simply taking the ratio of the resistance values.

If we use different colour filters ahead of the photometer tube, or even put two photometers with different colour filters side-by-side, we can measure colours.

| Top of the Page | Back to the MainPage | to my HomePage |

last update: March 2010 J.Köppen