I need it white! What do I do???

So you’ve got a device and you need to make it get from point A to a lot of other points with a minimal loss and as diffusely as possible. What does one do? Well, obviously, you paint the reflecting surfaces white, with a surface coating that is ‘white’ and matte- that’s simple enough! Or is it?

Well, not really. There are a number of factors that must weighed before we start coating things. How much heat is your part going to take? Will it be exposed to high humidity? What wavelength range is the device being used over? The answer to each of these questions will determine the ideal coating for your device.**

There are a number of solutions to this problem. The traditional solution is to use a barium sulfate-based coating. The old war-horse is Eastman 6080, white reflectance coating, developed in the early 1970’s. There are a number of iterations of this coating out there (many of them developed by yours truly). Our version is called Avian-B^TM white reflectance coating (the –B, cleverly enough, is for barium sulfate). This is a water/alcohol-based coating with barium sulfate and a binder. It adheres well to most metal and plastic surfaces (with a primer coat) and is both highly reflective and highly diffuse. The coating is also easily applied with a spray gun or even an air-brush for small parts. It is really the standard (and traditional) white reflectance coating. The coating is quite effective over a wide wavelength range (250- about 1300 nm, although it can be pushed out to about 2000 nm) and is 98+% reflective over a good bit of that range.

The downside of barium sulfate coating is that it is quite fragile and the binder is water-soluble. In addition, humidity affects the reflectance, especially in the NIR (near infrared). So while barium sulfate is good, it’s not perfect for every application. There are some alternatives. One is our Avian-D^TM(-D for durable) white reflectance coating. Avian-D^TM is also a water-borne coating, but with a polyurethane binder and a different pigment system. This coating was originally developed for outdoor use but we’ve found that it has numerous uses in place of barium sulfate. It is much more tenacious and is harder to rub off. And the reflectance is essentially equivalent to barium sulfate. The major drawback is a shorter wavelength range (really from about 320-1250 nm). And it is not totally water-proof; it’s water resistant (in other words, it wets, but when it dries it retains its original properties).

If one needs truly waterproof white reflectance coatings, there are some options. Labsphere’s Duraflect™ (developed by the kindly folks at Avian Technologies in a previous life) is a solvent based coating with a dual pigment system, which is waterproof, but has a fairly narrow wavelength range of usage. A similar coating is available from the former Optronic Laboratory (now Gooch and Housego) known as Optilon II™. Neither of the aforementioned coatings are available for customer application, primarily due to their hazardous nature.

Now, suppose you have a system where you can’t use a coating. Maybe it’s a small part that you can’t get inside to spray the coating (these coatings all need to be applied in multiple- usually 15-20 coats) or the device will be seeing fairly high temperatures (say extended time at >100°C). What can you do? The solution is not a coating but a material, sintered PTFE, known variously as Spectralon™, Fluorilon™, and other trade names. They are all derived from work done by Vic Weidner at NIST in the early 1980’s but extended upon and patented by the author in the late 1980’s. We’ll cover sintered PTFE in the next blog offering. It’s a long and fascinating story…

** by devices, we mean integrating spheres, boxes, cavities, and any myriad of other optical components- baffles, port plugs, port frames, etc.

Summary Table of Diffuse White Materials Properties

Name	Useful Wavelength Range	Useful Temperature Range	Water Fastness	User applied?
Avian-BTM (BaSO4+PVA)	250nm – 2μm	-40°C to +90°C	Low	Yes
Avian-DTM (polyurethane and proprietary pigment)	320nm – 1.25μm	-40°C to +100°C	Good	Yes
DuraflectTM (solvent based)	350nm-1.2μm	up to 80°C	High	No
FluorilonTM, SpectralonTM (Sintered PTFE)	250nm-2.5μm	> 250°C	High	N/A

(All trade names are trademarked to their respective owners.)

What We Measure. Part 2: Transmittance

Last month we reviewed the first of the basic units we measure: reflectance. We presented a simplified but rigorous definition of the way we quantify the flux reflected from a surface. In a similar way, this post addresses transmittance, which describes the flux that passes through an object, in particular if the spectral properties of the light have been affected. Science words for asking "is the sample colored?"

There are several applications for transmittance, the most common being quantifying the color of filters. Most likely you are viewing this posting with an LCD monitor, and each pixel is actually a tiny back-lit triplet containing a red, green, and blue filter. The selection of these filters was a careful engineering decision based on the output of the backlight and many other factors. Filters serve this and many many other roles in science and technology, but we are getting ahead of ourselves.

The basic equation for calculating transmittance is almost the same as that for reflectance:

Similar to what we defined for reflectance, we use Φ to indicate an amount of light, the subscripts t and i for transmitted and incident light. The diagram below shows the flow of light through the system. We have a light source, a beam of incident light passing through the sample, and then a detector measuring the amount of transmitted light.

Configuration for transmittance measurement of the sample.

Configuration for transmittance standardization.

The good news with transmittance is that standardizing the instrument is a bit easier than with reflectance. To measure the incident light Φi we need only remove the sample and make a measurement. The incident light passes through to the detector unaffected. We assume Φi=Φt and the standardization is complete. To be strictly correct,we need to compensate for the detector dark signal Φd and the transmittance of the standard Ts:

.

For many applications. Ts is 1.0 since our reference standard was simply air. For measuring liquids, a clear glass cuvette is used. For the standardization step, a cuvette is installed, usually filled with the carrier in your sample (water or a solvent) and then measured for Ts. In this case Ts can be slightly less than 1.0.

These measurements are quite straightforward so long as the sample is non-scattering. That is, it does not affect the direction of light travel, only the spectral properties of the light (via absorption). For scattering samples, we will also need to consider the measurement geometry. That is, the optical configuration. But that will be another posting.

What We Measure. Part 1: Reflectance

Welcome to the first Spectroscopy at Avian Technologies blog post.  Over the next few months, my colleagues and I hope to give you an idea of what we are all about when it comes to measurements — instruments, capabilities, traceability, uncertainties, and anything else you would like to discuss. We hope your comments and questions will guide where we go with this blog.

First of all, we’d like to review what we have for measurement capabilities. The material properties we measure are reflectance and transmittance. The specific definitions can be complex and confusing, but we will present an explanation at the level of the non-expert in the spectroscopy and color measurement communities. Reflectance is the more complex measurement, and this post will contain technical details explaining what it is. Measuring transmittance is a simpler procedure, but it comes with its own set of complications that we will cover in turn.

So, reflectance is simply a ratio: light reflecting off an object divided by the light incident on the object.  We use Φ to indicate the amount of light, the subscripts r and i for reflected and incident light. Sorry for the math, but a little bit is going to be required:

What’s so complicated about that? In principle, nothing, but consider this graphic showing one common arrangement of light source, sample and detector. We have some white light incident on the blue sample, and the blue light reflects off. We position a detector to measure the reflected light. The problem is that besides measuring the reflected light — the numerator of the equation — we also need to measure the incident light for the denominator. That would require positioning the detector where the sample is, or someplace else in the optical path. For higher-end or research instruments this is often possible, but for most devices this is not practical and would be too expensive to enable. 

How then to we measure reflectance, when the vast majority of instruments cannot even measure the simple values in our reflectance equation? We need some help from others. Usually that help comes from the manufacturer of the instrument. In any case, what we need is a sample that is of known reflectance. The measurement community has a special word for such samples: standards; in this case reference standards. 

Imagine you are trying to make accurate voltage readings. You purchase a battery that always produces exactly 5V.^‡ In the morning, you hook up this perfect battery and adjust the voltmeter so it reads 5V. An advanced voltmeter might also have the ability to dial in 0V. Between these two calibration steps, your meter can now accurately measure voltage. When we do the equivalent of the 5V and 0V measurements in our calibration procedure, we are measuring the reference standard and a black standard. The final form of reflectance is this:

Here Φ_d is the dark measurement and the reference reflectance data R_s.

In a real instrument, we cannot measure light quantities directly, but we infer the amount of light from the detector signal. This signal (current or voltage) is digitized and the processed in the control software.

The above measurement is carried out at every point in the spectrum. Depending on the application, this could be 250nm to 2500nm sampled every 1nm or 400nm to 700nm sampled every 10nm. Or mid-IR all the way out to 20µ. And from the reflectance spectrum, derivative values can be calculated, such as color, whiteness, and more. Or you can infer chemical composition, fading, and even counterfeit products! But that is another column...

‡ You may well ask, “how close to 5V does it have to be for me to say exactly 5V?” Good question. That depends on the uncertainty of the measurement process. We will cover that in our discussion about Traceability.

Welcome

The goal of this blog/FAQ is to foster interactive discussions across the spectroscopy community. There are many common (and many uncommon!) questions that we seem to repeatedly field for customers. We would like to make this information more directly available to you. More informed customers and colleagues make our jobs more productive and enjoyable.