How to Use Reflectance Spectroscopy to Study Solid Samples

Reflectance spectroscopy is a powerful method to analyze diverse types of materials and environments. It works by measuring the light that bounces off the surface of a solid sample, while the rest of the light is either absorbed, scattered, or passed through. This interaction gives information about the composition of the sample.

In this article, we will explain some of the basic principles of reflectance spectroscopy, describe the common components and procedures of a spectrometer system, and show an example application where reflectance spectroscopy is used to check the quality of a product.

Specular and Diffuse Reflectance

The way light reflects from the surface depends on how smooth or rough it is. Surfaces that are exceptionally smooth or shiny, like mirrors, optical parts, thin films, and some metals, have specular reflectance, which means the light reflects in the same direction as it came from (Figure 1). Surfaces that are rough or matte, like paper, powders, and grains, have diffuse reflectance, which means the light reflects in all directions (Figure 1). Some surfaces, like plastic beads, have both specular and diffuse reflectance, which is called total reflectance.

It is important to know the difference in reflectance types of the samples that need to be analyzed, because it affects how to set up the best spectral system to measure them.

Figure 1. The choice of sampling device in a reflectance measurement setup depends partly on the surface characteristics of the sample, as this illustration demonstrates.

Reflectance measurement system

You can customize your reflectance setup for different applications by choosing different components - spectrometer, light source, sampling optics and more - that work well together (Figure 2). Here are some things to think about:

Figure 2. This modular reflectance spectroscopy setup includes a spectrometer, tungsten halogen light source and reflectance probe. Setups like this can be adjusted easily to switch out components depending on measurement needs.

Diverse types of light sources can be used for reflectance spectroscopy. Deuterium-tungsten light sources are an excellent choice because they have a stable and smooth output from 190-2500 nm, and they can use the UV and visible parts of the spectrum separately. They also cover the NIR range, but with less intensity. Before taking measurements, make sure to warm up the light source for 15-20 minutes to get the best results. Also, DO NOT turn off the light source to take a dark measurement; use the shutter instead.

Spectrometer

For different research applications, you can use a general-purpose spectrometer, but there are also special models for high-speed applications (for example, sorting parts on a production line) and other industrial settings. You can also use many small spectrometers in devices for manufacturing as a part, subsystem, or complete, automated system.

When you choose a spectrometer, make sure to choose a detector and grating that match the wavelength range you are interested in. There are good options for UV, visible and NIR reflectance applications.

Light Sources

The most important thing about choosing a light source for reflection is to find one that has strong output over the wavelength range you are interested in. For color analysis or when you want to measure something like the human eye, the light source needs to cover 380-780 nm. For chemical composition of organic material, near-infrared or infrared light will give you more information. In most cases, a narrow light source will not give you enough useful spectral response, so you can ignore lasers and most LEDs.

For applications in the visible wavelength range, a tungsten halogen light source is great for reflectance at visible wavelengths, and for sorting or color applications. You should choose models that have a high-power bulb and a built-in shutter (this is useful for taking dark measurements without changing your setup).

Sampling Optics

The sampling accessory you need depends on the kind of samples you want to measure and the type of reflectance you want to capture. Here are some common options:

A reflection probe is useful for quick measurements and for small samples. Reflection probes can measure both specular and diffuse reflectance and can be adjusted with a probe holder to get the best angle. One drawback is that most reflection probes only capture part of the reflected light because they use the same direction for illumination and detection. Measurements made with a reflection probe are relative measurements.

An integrating sphere is a good option if the sample has a rough surface like brushed metal, grains, or powders. An integrating sphere can see all the reflected light from different angles, giving more accurate (and absolute) reflectance measurements. An integrating sphere can also measure curved surfaces or the color of small objects.

An integrating sphere has a special interior surface that diffuses the light. The light enters through a small hole and bounces around inside the sphere until it is uniform. A fiber at 90° to the hole then samples a small part of the light and sends it to the spectrometer. A barrier in front of the fiber helps block any direct light from the hole.

Collimating lenses are another option for reflection sampling. The lenses can be connected to optical fibers to customize the angle of incidence and collection. You can measure specular or diffuse reflectance this way, but it is cheaper and more time-consuming. You need to align the lenses carefully to avoid beam divergence and get good signal. You also need extra equipment to hold the lenses and the sample in place.

No matter what sampling optic you choose, make sure to keep the same sampling geometry in your setup - i.e., keep the angle and distance from the probe or sphere to the reference and sample the same for every measurement.

Reflectance Standard

Reflectance measurements are a ratio of the reflected light spectrum to the incident light spectrum. Since you cannot collect all the light that hits the surface, you need to measure the reflectivity relative to a reference standard.

There are different types of reflectance standards available, with reflectivity from about 88%-98% across UV-Visible-NIR wavelengths. Reflectance standards only give absolute measurements if they have NIST-traceable calibrated values. Some reflectance standards only give relative measurements. This means they are almost equally reflective at all wavelengths, no matter the angle of collection. Diffuse reflectance standards (Spectralon® and PTFE are common diffusers) are a good option for use with probes, lenses or integrating spheres.

If you lose your diffuse reflectance standard or it gets too dirty or damaged to use, do not use a white piece of paper instead. Paper is not as white as it looks. A piece of Styrofoam™ will work much better. It has diffused reflection and has even reflectivity across the visible range. Just remember to later measure the Styrofoam against a proper reference standard and correct all your spectra accordingly.

Specular reflectance standards are more versatile. You can use them for both high-reflectivity samples (very shiny surfaces) and low-reflectivity samples (e.g., anti-reflective coatings, blocking filters and substrates). Specular standards may be made of materials such as glass or an aluminum mirror on a fused glass substrate.

A reflectance application

The quality of the black coatings on printed circuit boards (PCBs) depends on how they reflect light in different ways. This is important for PCBs in optical devices, where unwanted light can affect how they work.

Ideally, a black surface should not reflect any light at all. However, there are many kinds of black surface coatings that have different levels of absorption and reflection depending on the wavelength of light.

To measure this, a spectrometer (350-1000 nm) with a light source and a probe was used to get the reflection spectra of PCBs with three kinds of black paint (matte black, shiny black and blackboard paint).

A grayscale standard (~20% diffuse reflection) was used as a reference because these samples did not reflect much light. The integration times were between 20-120 milliseconds, with a boxcar of 5 and 20 scans to average. A ring stand held the probe holder for the measurements and a black hood blocked the ambient light. The probe holder was set at 45° (diffuse reflection) and 90° (specular reflection) to the coated surface for the measurements. A grayscale standard (~20% diffuse reflection) was used as a reference because these samples did not reflect much light. The integration times were between 20-120 milliseconds, with a boxcar of 5 and 20 scans to average. A ring stand held the probe holder for the measurements and a black hood blocked the ambient light. The probe holder was set at 45° (diffuse reflection) and 90° (specular reflection) to the coated surface for the measurements.

The diffuse reflection data (Figure 3) shows that the flat and glossy black paints have a similar response in the visible range. This means that these paints would look very similar in color to the eye, even though one of them reflects more light than the other.

Figure 3. With diffuse reflectance measurements, differences in spectral response for PCBs with different surface coatings become more pronounced at higher wavelengths. Additional analysis could confirm the source of this difference.

The NIR shows that the flat and glossy black paints are different, which means that the gloss may come from a compound that reflects NIR. The chalkboard paint seems to reflect more light than the other black paints. But this is only based on the reflection probe at 45°.

This example shows how a simple spectrometer setup can measure the reflection properties of black coatings on PCBs. The spectral data from these easy measurements can help choose the best coating for the optical bench to reduce unwanted light and improve the device performance.

Spectroscopy and its applications have advanced a lot, so a simple reflection measurement like this can be used for similar applications. With the flexibility of reflection spectroscopy, today's spectrometer providers can help users in different markets solve problems from research and science applications to industrial and OEM solutions.