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

  • #1
aferrie
6
2
TL;DR Summary
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.
 
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  • #2
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)?
 
  • #3
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?
 
  • #6
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?
 
  • #7
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.
 

1. What is an integrating sphere and how does it measure reflectance?

An integrating sphere is a spherical cavity with a highly reflective interior surface. It is used to measure the reflectance of non-planar objects by directing light onto the surface of the object and collecting the reflected light with a detector. The sphere ensures that the light is evenly distributed over the object's surface, allowing for accurate measurement of reflectance.

2. How does the shape of the object affect the measurement of reflectance with an integrating sphere?

The shape of the object can affect the measurement of reflectance with an integrating sphere because the light may not be evenly distributed over the entire surface. This can result in variations in reflectance measurements at different points on the object. It is important to position the object in the center of the sphere and rotate it during measurement to ensure accurate results.

3. Can an integrating sphere measure the reflectance of both diffuse and specular surfaces?

Yes, an integrating sphere can measure the reflectance of both diffuse and specular surfaces. The sphere's reflective surface ensures that light is evenly distributed over the object's surface, allowing for accurate measurement of both types of surfaces.

4. How is the reflectance value calculated from the measurements taken with an integrating sphere?

The reflectance value is calculated by comparing the intensity of the reflected light from the object to the intensity of the incident light. This is done using a spectrophotometer or other light measuring device. The reflectance value is expressed as a percentage, with 100% being a perfectly reflective surface and 0% being a perfectly absorptive surface.

5. Are there any limitations to using an integrating sphere for measuring reflectance?

While integrating spheres are a useful tool for measuring reflectance, there are some limitations to consider. The size of the sphere limits the size of objects that can be measured, and the shape of the object can affect the accuracy of the measurement. Additionally, the reflective surface of the sphere may not be perfectly uniform, which can also impact the accuracy of the results.

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