spectrophotometers, spectroradiometers, and the i1 pro - AVS Forum
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Old 12-05-2013, 09:06 PM - Thread Starter
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I've noticed that people conflate these terms a lot, so I thought I'd share the proper definitions:

A spectrophotometer measures spectral reflectance of an illuminated object.

A spectroradiometer measures spectral radiance of an emissive source.

This is a somewhat strange definition, since the terms "photometric" and "radiometric" are usually distinguished based on the luminous spectral efficiency function of the human eye. In the case of these instruments, however, the terms "photometer" and "radiometer" do not seem to reflect this distinction.

Based on the above definitions, the i1 pro is technically both a spectrophotometer and a spectroradiometer. However, X-Rite does not classify the instrument as a spectroradiometer. After a lengthy discussion with the head of their tech support, it turns out that the reason for this is because the i1 pro does not meet international standards and certifications of a spectroradiometer. I'm assuming this has to do with the quality of the dispersing optics, and/or wavelength sampling abilities. They actually consider the i1 pro to be functioning as a colorimeter when measuring emissive displays, as its express use is for deriving colorimetric data (X Y Z), rather than spectral radiance data (even though it is deriving this data from spectral radiance data rather than directly from the source, as a true colorimeter would).

However, there are industry professionals who refer to the i1 pro as a true spectroradiometer.

Thought I'd share this, as it brings up a couple interesting points:

1) Do you classify an instrument based on how it achieves its measurements, or on the units it explicitly outputs?

2) Do you classify an instrument based on its fundamental operation (the i1 pro certainly is a spectroradiometer in the strict sense of the word), or based on its ability to meet international standards of certification?
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Old 12-06-2013, 09:50 AM
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To us (SpectraCal) anything that measures raw spectrum and then uses a CMF to derive XYZ as it's base internal measurement is a spectroradiometer or spectro for short. Add the ability to illuminate and you have a spectrophotometer. You are right the X-Rite i1Pro/i1Pro G2 are both a spectroradiometer and a spectrophotometer depending on the the mode used. Most spectro's are one or the other.

Devices that use filters be it 3 to 40 that mimic CMF are a colorimeter. Most are tristim's 3-6 filters but some like the KM CS-200 have 40 filters to create a "Spectral Fit Method".

For us we break it down to two classes spectro's and tristim's. Spectro's give us SPD and tristim's give us XYZ.

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Old 12-06-2013, 11:10 AM
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Old 12-06-2013, 11:11 AM - Thread Starter
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Thanks Derek, that's good to know. I guess X-Rite distinguishes between a scientific definition and a commercial definition.
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Old 12-06-2013, 11:52 AM
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I just use Spectro when I reference the i1Pro (1/2) and have for a long time.. for our purposes, it's fine.

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Old 12-06-2013, 12:00 PM - Thread Starter
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Quote:
Originally Posted by turbe View Post

I just use Spectro when I reference the i1Pro (1/2) and have for a long time.. for our purposes, it's fine.

Yea, most people do. I was just interested in the "official" definitions.

I first came across the proper distinction in Yoshi Ohno's chapter in Colorimetry: Understanding the CIE system.

excerpt:

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Old 12-06-2013, 12:17 PM
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You got a bit more out of X-Rite in regards to not meeting international standards/certifications for spectroradiometer.

Perhaps tracking down specific info on those would be of value of this thread...

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Old 12-06-2013, 12:28 PM - Thread Starter
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Quote:
Originally Posted by turbe View Post

You got a bit more out of X-Rite in regards to not meeting international standards/certifications for spectroradiometer.

believe me, it took many back and forths before I got that critical piece of information, although the person was very patient with my questions.

Quote:
Originally Posted by turbe View Post

Perhaps tracking down specific info on those would be of value of this thread...

I found this, from NIST, which appears to list the minimal requirements for certification of a spectrophotometric components and function. Will look around for a similar document for spectroradiometers.
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