This page last changed on Feb 07, 2005 by sbannasch.

I need answers to these two questions:

  • What ranges do we need to measure and at what resolutions?
  • What sources of light will we be measuring?

In order to answer this question:

  • What units should we be using with the light probes?

I had been assuming that we would use lux however some of the light probes offered by the manufacturers don't actually measure lux they measure a more broadband radiant flux.

I recommend that we report values in lux and that the activity designers make sure that students are not asked to compare measurements from light sources with different spectral distributions.

A larger issue (that Ed H is dealing with) is how to measure the high levels of illumination specified in his investigations with the very low maximum light levels measured by the Pasco and TI probes.

details below

Manuf. model

The Verrnier, CMA-Coach, and Data Harvest light probes have a photopic response. The Coach light probe has a single range and a maximum measurement of 150,000 lx. The Vernier probe has three ranges: 0-600, 0-6000, and 0-150000. Both of these probes are equivalent to the TEEMSS1 light probe. The built-in light sensor in the EasySenseQ as well as thehas two ranges: 0 ? 1,000 and 0 ? 100,000 lux. I don't yet know what type of sensor they use.

The Pasco and TI light probes have the typical spectral response of a photodiode which starts at 300 nm, peaks at 1000 nm, and stops at 1100 nm. They report their data in relative irradiance. The Pasco probe has three ranges and a maximum light level of 500 Lux. The TI probe measures a maximum irradiance of 10 W/m2. At 555 nm this is equivalent to 6830 lx.

I don't yet have any information on the light sensor used in the Fourier EcoLog XL.

A photopic response is one that is spectrally weighted to match the spectral response of the human eye. Lux is a quantity normally used to measure visible illumination. 1 W/m2 of light at 555 nm (green) is 683 lx, while the same intensity at 650 nm (red) is 73 lx, and at 470 nm (blue) 62 lx.

We can't make a perfect conversion from irradiance using the TI and Pasco probes to Lux however measurements taken by a single type of probe are comparable as long as the type of light source stays the same.

I can create a rough calibration to convert relative irradiance to lux for these probes as long as we select a default type of light source.

FYI: a very good web resource on light measurements:

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