Measuring reflectance of a non-planar object w/ an integrating sphere

Click For Summary

Discussion Overview

The discussion revolves around the measurement of reflectance (both diffuse and specular) of non-planar insect specimens using an integrating sphere. Participants explore the implications of using a non-planar sample and the potential impact on measurement accuracy, particularly concerning the orientation of the sample and the illumination setup.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses concern that using a non-planar object may lead to incorrect reflectance measurements due to the varied angles of specular reflectance, suggesting a large integrating sphere to minimize losses.
  • Several participants seek clarification on the specific type of reflectance measurement intended, questioning whether it is directional-hemispherical, hemispherical-directional, or full bi-directional reflectance (BRDF).
  • Another participant confirms the need to measure directional-hemispherical reflectance based on definitions provided in external sources.
  • There is a suggestion that an integrating sphere could be suitable for the measurement, with emphasis on the importance of designing the sample mount for proper orientation relative to the illumination beam.
  • A participant mentions the use of an aperture to ensure the illumination beam is slightly larger than the specimen and discusses methods to manage the beam's interaction with the sample.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the specifics of the measurement type, as there are multiple interpretations of the terms used. The discussion remains unresolved regarding the optimal approach for measuring reflectance of non-planar objects.

Contextual Notes

Participants reference definitions from external sources to clarify the types of reflectance measurements, indicating potential limitations in understanding or agreement on terminology.

aferrie
Messages
6
Reaction score
2
TL;DR
Are there any potential issues with measuring the reflectance of a non-planar object (e.g. a ball bearing or an insect specimen) using an integrating sphere?
I am designing an experiment to measure the reflectance (diffuse+specular) of insect specimens, and I plan to use an integrating sphere to collect the measurements. Typically, I have seen that reflectance measurements with integrating spheres are done with planar (i.e. flat) sample objects. So, I am curious if a non-planar sample object would produce an incorrect measurement.

My main concern is that the specular reflectance of the sample is now contained over a variety of angles, so orienting the sample at an 8deg incidence angle with respect to the source (as shown below) will not redirect all of the specular signal within the sphere. My proposed solution is to obtain a large integrating sphere so that the port fraction is low (i.e. around 0.3% assuming a port diameter of ~13mm and and integrating sphere diameter of 200mm). This will not eliminate all of the losses of the specular signal, but it will minimize them such that the reflectance measurement is accurate.

1699402598293.png

Image source

For reference, the reflectance measurement needs to be taken with each specimen fully illuminated.
 
Science news on Phys.org
It's not clear what you are trying to measure- the image you included shows a measurement of the directional-hemispherical reflectance, but your final sentence seems to indicate you want to measure the hemispherical-directional reflectance.

Or, do you want to measure the full bi-directional reflectance (BRDF)?
 
Andy Resnick said:
It's not clear what you are trying to measure- the image you included shows a measurement of the directional-hemispherical reflectance, but your final sentence seems to indicate you want to measure the hemispherical-directional reflectance.

Or, do you want to measure the full bi-directional reflectance (BRDF)?
Can you elaborate on what you mean by directional-hemispherical reflectance vs. hemispherical-directional reflectance?
 
aferrie said:
https://nvlpubs.nist.gov/nistpubs/Legacy/MONO/nbsmonograph160.pdf

Based off of these definitions, I need to measure the directional-hemispherical reflectance.
Then I think an integrating sphere will work well for you; spend a little time designing the sample mount so that you can orient the sample with respect to the illumination beam as you like. I'm still a bit confused about "the reflectance measurement needs to be taken with each specimen fully illuminated."- is the illumination beam larger diameter than the object?
 
Andy Resnick said:
Then I think an integrating sphere will work well for you; spend a little time designing the sample mount so that you can orient the sample with respect to the illumination beam as you like. I'm still a bit confused about "the reflectance measurement needs to be taken with each specimen fully illuminated."- is the illumination beam larger diameter than the object?
Yes, the beam will be slightly larger than the object (I will use an aperture to limit the beam diameter). To eliminate the section of the beam that is not illuminating the insect, I plan to either use a transparent mounting slide with a light trap behind it or a slide made out of a highly absorptive material.
 

Similar threads

Undergrad Help with sourcing a specific type of integrating sphere
  • · Replies 2 ·
Replies
2
Views
3K