Understanding a Reduced Light Output Situation

[vjop]

I am looking for some input on a phenomena that I am experiencing and do not understand. I have an illumination system where I am emitting a colored light through an identically colored lens as well as sending the same colored light through a clear lens. Both lenses are made from the same type of polycarbonate material. When I measure the photometric flux, the colored lens combination produces more flux than the clear lens combination. This is counter intuitive. I would have envisioned the colored lens as having more "stuff" that gets in the way of the light than the clear lens. Any ideas? Thanks.

 

[IRstuff]

What wavelength is the source? Is it broadband?
What are you using to do the measurement?



TTFN

 

[vjop]

The wavelength of the light source is nominally 590nm. The testing is being done in a light tunnel using a photometer and radiometer by International Light Inc.

 

[IRstuff]

How much difference are you seeing?
Have you tested this against other sources, other wavelengths?

Are the lenses AR coated?
Do you have spectral data for the lenses?
Are they flat?
Is the measurement in an otherwise well-lit room, or are the lights off?

TTFN

 

[vjop]

I am seeing a difference of approximately 25%.
I have not tested against other sources though it is possible for me to do so.
The lenses have no coatings.
I have no spectral data on the lenses.
They are not flat.
The measurements were done in a room without any light.

 

[IRstuff]

Do both instruments measure the same thing without the lenses?

TTFN

 

[IRstuff]

How big are the apertures of the photometer and radiometer?

Did you verify that the beam fits entirely within the apertures?

TTFN

 

[vjop]

The photometer (sensor) - radiometer (readout device) combination measure the same light output without any lens in this case because the only difference is the lens. I am reusing the electronics and lighting package.

The photometer is approximately 1.5" in diameter.

I am only sensing a portion of the overall output since this is an illumination system in combination with a small sensor.

 

[IRstuff]

So the lenses basically take something like a red LED and spreads it out?

How sure are you that both lenses are identical in magnification and size. Are they simple plano-convex lenses?

I'm sort of running out of things to pick at. The possible scenarios are thus:

> The lenses might actually have different transmissions. 25% is a bit subtle to tell when there's a massive color change, i.e., a 75% transmission lens placed next to a 100% transmission is noticeable.

> The lenses have different effective magnifications so that you're not collecting the same solid angle's worth of the LED output. A way to check, if you haven't done so, is to run the lens "backwards" and attempt to focus the LED output back down to a point and measure the distance to the focus as a check on the focal length of the lenses.

TTFN

 

[vjop]

The lenses are made from the same mold, but perhaps there are subtle differences where the effective focus point has changed. I'll try moving the light source closer or farther away. The lens shape is intended to spread the light out in a uniform intensity pattern.

Unfortunately, I can not run the light backwards as you suggest because of the shape of the lens. It is a good idea though.

I have a strong belief that there is something in the material that is reducing light transmission, but I have no way of identigying what it is.

 

[IRstuff]

Well, you could try putting a red laser pointer through them and measuring the lens transmission that way.

Another possibility is measure the raw blanks for transmission. Using a "witness" sample is often done when the actual product cannot be readily tested.

TTFN

 

[vjop]

I think I will try a witness sample. I may also have a calibrated light source to use as well. Thanks for the tips.

 

[IRstuff]

You might be able to rent a spectrometer, use a broadband light source and compare the spectral transmissions.

I've looked a couple polycarbonate websites with spectral curves and all appear to be relatively flat spectrally, although these sites show only about 82% transmission at 590nm:

http://www.nationwideplastics.net/antistatic_polycarbonate.htm

http://www.fresneltech.com/graphs/polycarbonate_graph.html

So it might be possible that the red PC is simply higher tranmission, although that would seem to be unlikely to be much higher than about 90%, which would result in maybe 10% tranmission difference.

TTFN

 

[vjop]

Your observations on the transmission efficiency matches up with what I have found.

I do have a colorimeter which may work as a spectrometer.