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.
 

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

What is an integrating sphere and how does it work?

An integrating sphere is a spherical device coated internally with a diffuse white reflective material. It works by capturing and diffusing light uniformly across its interior surface, allowing for the measurement of the total reflectance or transmittance of an object. Light entering the sphere is scattered multiple times, creating a uniform light field, which can then be measured by a detector placed at a port in the sphere.

Why is an integrating sphere used for measuring the reflectance of non-planar objects?

An integrating sphere is used for measuring the reflectance of non-planar objects because it can capture and integrate light reflected in multiple directions, ensuring that the geometry of the object does not affect the measurement. This is particularly important for non-planar objects, which can scatter light in unpredictable ways. The integrating sphere averages these variations, providing an accurate and consistent reflectance measurement.

How do you prepare a non-planar object for reflectance measurement in an integrating sphere?

To prepare a non-planar object for reflectance measurement in an integrating sphere, ensure the object is clean and free from any contaminants that could affect reflectance. Position the object at the appropriate port of the integrating sphere, ensuring it is securely mounted and properly aligned to avoid shadowing or blocking the light source. The object should be placed in a manner that maximizes the exposure of its surface to the incident light while minimizing any gaps or obstructions.

What are the key factors to consider when measuring reflectance with an integrating sphere?

Key factors to consider include the size and shape of the object, the wavelength of light used for measurement, the uniformity of the sphere's coating, and the positioning of the object within the sphere. It's also important to calibrate the integrating sphere with a known reference standard to ensure accurate measurements. Additionally, the sphere's ports should be appropriately sized and positioned to minimize light loss and ensure consistent measurements.

How do you interpret the data obtained from reflectance measurements using an integrating sphere?

The data obtained from reflectance measurements using an integrating sphere typically include the total reflectance, which is the ratio of the reflected light to the incident light. This data can be used to determine the material properties of the object, such as its color, gloss, and surface texture. By comparing the reflectance data to known standards, one can also assess the quality and consistency of the object's surface. The results are often presented as a percentage, indicating how much light is reflected by the object at different wavelengths.

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