Photon Counting Terminology: Are We Really Counting? (original) (raw)

Posted on 2025-08-25 as part of the Photonics Spotlight (available as e-mail newsletter!)

Permanent link: https://www.rp-photonics.com/spotlight_2025_08_25.html

Author: Dr. Rüdiger Paschotta, RP Photonics AG

Abstract: Photon counting is an interesting concept, but the term is often inappropriately used. Better terminology would avoid some confusion in science and even more so in technology.

Content quality and neutrality are maintained according to our editorial policy.

In science and technology, clear terminology is something very desirable to avoid wasted effort and mismatched expectations. Yet some usages become entrenched and hard to eradicate. A noteworthy example is the phrase photon counting.

Imagine a young boy who raises a hand each time he sees a cat. Is he counting cats? No, unless he is able to report their number!

But why then do we consider photodetectors as photon counting detectors, if they do nothing more than signaling incoming photons, e.g. with a digital output pulse for each, not caring about their number? Is that appropriate, or should we change our use of these words?

What Exactly is Counting?

Everyday language is often less clear than the language of science, where clarity matters most. In this case, however, everyday language is pretty consistent: With counting, we nearly always mean determining numbers. An exception is that we sometimes say that we count something as belonging to a certain category, where consider would be more clear. Also, we may say that an aspects counts in the sense that it matters. But I think never we merely mean registering when saying counting. Take the above example of the boy registering and indicating cats, but not counting them.

Already in the early literature on single-photon detectors, the term photon counting started to occur quite frequently even when it was only about detecting in the sense of registering photon events. Maybe people thought that if you are able to register single photons, it is a trivial extension to count them: Just add an electronic counter. And maybe they already thought about benefits of counting photons in applications, without actually doing it already.

Still, when we talk about a counter, we usually assume it is a device which is able of true counting: receiving input signals and delivering their recorded number.

Can Confusion Arise?

Some people hate discussions like this, saying it is only about terminology, not something of substance — something for pedantic people to waste their time. However, it belongs to the foundations of science that we should really care about greatest clarity, avoiding confusion wherever possible, and in technology this is certainly also useful. Being pragmatic rather than pedantic, the core question is whether inappropriate wordings have the potential to create confusion. And I think this is given in our case.

Finding Products Which Do Counting

Imagine someone searching for a device which can count photons in a true sense, i.e., report or display its numbers. It can then be rather frustrating if many of the found suppliers claiming to offer photon counting detectors in fact only have detectors which can signal photons, for example by generating a digital output pulse for each registered photon.

In a product directory, you would certainly prefer to have one product category single-photon detectors and another one called photon counting equipment (where the latter also includes related devices such as electronics). This is exactly what I now have prepared for the RP Photonics Buyer's Guide.

Understanding Technology

Is another example, you can read many reports on the use of photon counting detectors for medical imaging, namely computed tomography (CT). You may then wonder what type of detectors they really use: These are semiconductor-based detectors, which can now even resolve photon energies to some extent, which happens to be very useful for high-quality imaging with minimum radiation dose. But do these devices count photons? No, they don't — counting is done by additional electronics of relatively trivial kinds. So a newcomer to this field would be better served by being told that they use single-photon detection with energy resolution. But the whole field is already widely announced as “photon counting computed tomography”, and if you Google for “photon counting”, a large share of the results is about that.

Understanding the Concept of Counting

In science itself, one may think there is no serious potential for confusion — not only because people with some experience in the field know about the use of terms (and who cares about the others?!), but also because the actual counting is often quite obviously not the point of interest. Technically, the registering of single photons is quite demanding, while counting with some suitable digital electronics is comparatively trivial, even at the highest speeds the detectors allow for. However, by only thinking along these lines, one overlooks that the concept of counting has additional important implications for applications. For example, an interesting question is how counting photons compares to using analog photocurrents in terms of achievable performance. As my encyclopedia article on photon counting explains, photon counting has important advantages but also limitations, and these aspects really do not focus on the registering process but rather on the concepts of counting vs. analog integration.

By using the term “counting” only where appropriate, readers will be more likely to pick up the relevant thoughts. For example, the RP Photonics Encyclopedia now talks about counting where it should, and there explains the concept properly.

Problems with Multi-photon States

Another interesting aspect is under which circumstances photodetectors can count photons. It turns out that most single-photodetectors are not able to distinguish one or several photons, let alone to reliably tell their number, if they all come within a short time interval. In practice, that happens with multi-photon quantum states of light and also for short light pulses. Only a few detector types have that capability, and it is common (and appropriate) to call those photon-number resolving detectors.

So it is somewhat odd that most detectors described as photon counting detectors can exactly not count multiple photons coming simultaneously or within a short time interval! For true counting, they can be used only with single-photon states, which fortunately we have in many cases of practical interest. For example, that often occurs when monitoring week fluorescence. For judging which detector might be suitable in a certain case, some knowledge of quantum photonics is required.

Conclusions

I conclude that it would have been definitely better if researchers would have restricted the use of the term counting to cases where it is really appropriate. Especially in some technology contexts, that would have avoided some confusion. I think that we should finally start using more clear wordings, although clearly this cannot repair the literature and will thus have only limited benefits. At least, I don't think there is any good reason for simply ignoring such problems and not caring so much about clarity.

This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page and cite it, because its location is permanent. See also the RP Photonics Encyclopedia.

Note that you can also receive the articles in the form of a newsletter or with an RSS feed.

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.