I recently purchased an LT40 light meter. 

I made my purchase decision based on the downloaded spec sheet.

The spec sheet says the LT40 has +/-3% accuracy and measures white Led and standard lighting.

I did an illumination study using the LT40 of a Tennis facility that uses metal halide bulbs. 

The meter operated as expected. All should be good.

Or so I thought until I read the manual more carefully.

The manual that came with the meter states that the LT40 "measures white LED, fluorescent, metal halide, high-pressure sodium, and incandescent sources"

great! exactly what I need and expected.

The manual also states that the accuracy of the meter varies dramatically with some of the supported sources.

I did not expect this.

With which visible source(s) is the meter +/- 3% accurate?

With which source(s) is the meter +/- 8% accurate?

I am wanting to measure "LED, fluorescent, metal halide, and incandescent lighting sources" and expect the same accuracy for all sources.  Is the LT40 the right meter choice for my needs?



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The LT40 specifications for +/- 3% refer to the measuring accuracy of White LED lights.
The +/- 8% refer to all other types of light (fluorescent, incandescent, Sodium, etc.)

When measuring LED light you might ask why you need an LED light meter.

Typically, most people really only need to use a standard visible light meter.  Let me explain.

When you are measuring light for a human, any of our visible light meters will work.  This is because the sensor is designed to "see" the visible light spectrum that a human sees (Photopic light range).  LED light puts out some light in the Blue range but this is just outside of the human light response. So when "comparing" light sources (Sodium, LED, Fluorescent, etc.) you need only a light meter that can "see" light that a human can see.  It sounds like the LT40 is not really the meter you need.  Note: If you have to measure Low light levels for parking lots or dimly lit hallways or stairways, a light meter that has good accuracy in the Low light range would be best.  Check out the Extech EA3x series.

Any Questions?


Best Regards,

Steve Hyde

Extech Tech Support



Thanks for the clear response.

My primary application is outdoor Tennis lighting.

The 2 target sources are metal halide and led fixtures.

The footcandle range is 10 to 150.

Other than that I will be measuring led, incandescent, and fluorescent in my home.

I would like to confirm that the EA31 has a photopic accuracy of +/- 3% for each of metal halide, led, incandescent, and fluorescent within a 10 to 150 footcandle range?

thx for your help


Basically yes, however, always go by the accuracy specs in the User's manual.  The Datasheet is a marketing brochure.

The accuracy as listed in the manual is
Up to 1000 Fc it is +/- (3% of reading + 0.5% Full scale) and if greater than 1000 Fc it is  +/-(4% of reading +0.5% full scale).  This is for all types of lighting.



Thx.  I initiated an exchange of the LT40 for an EA31.

From the specs the LT40 would seem to have very close to the same photopic CIE matching accuracy for led light as the EA31. But worse accuracy for other sources.

For what sort of application was the LT40 intended?


The LT40 and LT45 were designed specifically to measure LED lighting.
It also measures outside the Photopic range, as it is able to measure also into the blue range of light that the LEDs produce (~410nm).



Since the LT40 measures light below the blue visible to most humans, it must have been designed to measure white LED lighting for machine sensing applications?  ( Possibly for factory line scanners, cameras, etc., instead of human sight )



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