Issues to consider when measuring L.E.D. light using an Extech light meter.

Measuring and evaluating L.E.D. light should be done on a comparative basis. Measure a known good sample and compare the measurements to your test samples.

All of the Extech light meters are calibrated for visible light at a color temperature (white balance) of 2856K (incandescent - ~560nm).

The spectral response of the sensor and filter is designed to meet C.I.E.photopic (human eye response curve).

L.E.D. light, depending on the color, is radiating at a different color wavelength than incandescent light.

Example –    ~red L.E.D. is 700-640nm, green is ~550-520nm, and blue is ~480-450nm.

A “white” L.E.D. can be either made up of Red, Green and Blue (RGB) L.E.D. light or they can use a phosphorus material to convert blue or UV light to a broad spectrum white light.  Both methods are not monochromatic but produce a range of wavelengths of light

An L.E.D., especially white, can also emit a slightly different light at different operating temperatures and currents.  So your test setup should consider these 2 factors for repeatability.

When trying to measure the light output of a L.E.D. you will notice from the supplied CIE photopic light response graph, that there is a roll off of the amount of light measured above and below the nominal human eye response (550nm).

The model 407026 Extech light meter has a built in calibration adjustment for various light sources. 

For all other Extech light meters, you can apply these correction factors to your measurements based on the type of light source.

As mentioned above, for L.E.D. light, you need to perform a comparative measurement process.