Transmission test problem

[Lu Wei]

Hi all,
I am doing some transmission measurements using spectrometer PerkinElmer Lamda 750. It uses halogen lamp. The sample is normal soda-lime float glass. I find a weird phenomenon: The result varies with the sample' position on the light path. Specifically, in the middle of the sample chamber where the light is more focused, the measured transmission is smallest; move the sample towards the entrance or exit, where the light spot gets bigger, the result gets bigger (continuously). The maximum difference is about 0.5%.

I cannot think of any reason caused this. The parallel light is ideal; yet near parallel light should be OK to use. The incidence angle should be the same regardless of the position. The reflectance and absorption should be the same regardless of the light spot size. Should they?

The optical diagram of could be the spectrometer could be found here:
http://www.perkinelmer.com/CMSResources/Images/44-74449BRO_LAMBDA-750-UV-Brochure.pdf

[Helpful person]

Post by Lu Wei
Hi all,
I am doing some transmission measurements using spectrometer PerkinElmer Lamda 750. It uses halogen lamp. The sample is normal soda-lime float glass. I find a weird phenomenon: The result varies with the sample' position on the light path. Specifically, in the middle of the sample chamber where the light is more focused, the measured transmission is smallest; move the sample towards the entrance or exit, where the light spot gets bigger, the result gets bigger (continuously). The maximum difference is about 0.5%.
I cannot think of any reason caused this. The parallel light is ideal; yet near parallel light should be OK to use. The incidence angle should be the same regardless of the position. The reflectance and absorption should be the same regardless of the light spot size. Should they?
http://www.perkinelmer.com/CMSResources/Images/44-74449BRO_LAMBDA-750-UV-Brochure.pdf
--
Regards,
Lu Wei
PGP key ID: 0x92CCE1EA

The only possibility that comes to mind is that there may be some wedge or power in your sample.

http://www.richardfisher.com

[Lu Wei]

Post by Helpful person
The only possibility that comes to mind is that there may be some wedge or power in your sample.

The sample is 0.55mm low iron soda-lime float glass, no coating, no any
structure. Both front and back surfaces are flat, and parallel I think.

[Phil Hobbs]

Hi all, I am doing some transmission measurements using spectrometer
PerkinElmer Lamda 750. It uses halogen lamp. The sample is normal
soda-lime float glass. I find a weird phenomenon: The result varies
with the sample' position on the light path. Specifically, in the
middle of the sample chamber where the light is more focused, the
measured transmission is smallest; move the sample towards the
entrance or exit, where the light spot gets bigger, the result gets
bigger (continuously). The maximum difference is about 0.5%.
I cannot think of any reason caused this. The parallel light is
ideal; yet near parallel light should be OK to use. The incidence
angle should be the same regardless of the position. The reflectance
and absorption should be the same regardless of the light spot size.
Should they?
http://www.perkinelmer.com/CMSResources/Images/44-74449BRO_LAMBDA-750-UV-Brochure.pdf

Could be defocusing caused by heating. The hot spot becomes convex.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

[Lu Wei]

Post by Phil Hobbs
Could be defocusing caused by heating. The hot spot becomes convex.

Unlikely, because the slit is set to 5nm, that is when measuring, only a
very small portion of spectra is passed onto the sample, and even if the
whole spectra is passed during aligning, the spot is not very bright or
hot to any extent that could curve the glass, for I can put my finger on
the focus.