Reflectance

Very smooth or shiny, mirror-like surfaces have high specular reflectance, in which the incident light reflects in the same direction. Rough or matte surfaces have diffuse reflectance, where the incident light gets scattered in all directions. Most surfaces exhibit a combination of specular and diffuse reflectance.

Ocean Optics modular spectrometers and accessories can be configured for a variety of diffuse and specular reflectance measurements, with options for lab use and feasibility or quality control and process monitoring. These are the core components to consider for most applications:

Spectrometers

A workhorse spectrometer like the Ocean SR or Ocean HDX is a good choice for most applications, although the Ocean HR high-resolution spectrometer and super-fast Ocean FX also are useful for reflectance and color measurements. NIRQuest+ spectrometers cover your NIR (900-2500 nm) needs.

Here are some other considerations in selecting a spectrometer for your spectrometer setup:

• Configure the spectrometer with a grating that matches the wavelength range of interest. We offer preconfigured versions of most spectrometers, with VIS-NIR wavelength range options (which vary by spectrometer model) best suited to most reflectance applications.
  
  
• For industrial applications, consider a complete solution. We have the tools and experience to help you measure reflectance and color in challenging environments. For example, we can integrate an Ocean HDX as a component, subsystem or complete system into your process line.

Light Sources

The most important thing about choosing a light source for reflection is to find one with strong output over the wavelength range of interest.  For color measurements or when making a measurement to mimic the human eye, the light source needs response from 380-780 nm. For chemical composition of organic material, near-infrared or infrared light will offer more information. Except in a very few specific cases, a narrow light source will not offer enough useful information, so lasers and most LEDs can be ruled out. 

• Visible light sources. The broad, smoothly varying output of a tungsten halogen light source is ideal for reflectance at visible wavelengths, and for sorting or color applications. The HL-2000 series is the choice here, with standard, long-life and high-power versions available. We recommend the HL-2000-HP-FHSA model, which has a high-power bulb (too much light is a nice problem to have!) and built-in shutter (handy for taking dark measurements without disturbing your setup).
  
  
• UV-visible light sources. Both deuterium tungsten and xenon light sources can be used to make a UV-Vis reflectance measurement, but each has its own advantages. Deuterium-tungsten light sources have a broad, smoothly varying spectrum and stable output from 190-2500 nm, and since their output comes from two different bulbs, the UV and visible portions of the spectrum can be used separately.    
  
  In contrast, xenon light sources have very jagged and pulsed spectra, though they have good output in the UV. Spectral averaging is necessary to get quality measurements. Xenon sources can be used for UV reflectance measurements in the field since they run on 12 VDC power. 

• NIR light sources. The HL-2000s have output into the NIR, out to about 2500 nm. Even though intensity decreases at the longer wavelengths, this effect is offset by higher sensitivity of the detectors in our NIR spectrometers at those wavelengths.

Sampling Optics

Are you measuring specular or diffuse reflectance? Flat surfaces or irregular shapes? Your choice of sampling accessory depends on various factors.

• Reflection probes. A reflection probe is great for making quick measurements and for applications where a small spot size needs to be sampled. Probes can be positioned to measure specular or diffuse reflectance and are compatible with our spectrometers and light sources (provided the probe fiber bundle matches the wavelength range of the light source). The downside is that the probe illuminates and detects from the same direction, so it only sees part of the reflected light. Measurements made with a reflection probe are relative. Also, probe holders are available for fixturing the probe at 90° or 45° to the surface.
  
  
• Integrating spheres. An integrating sphere is a good idea if the reflectivity of the sample seems to change at different viewing angles. This happens with the rough surfaces of samples such as brushed metal and plastic parts, fish scales, and grains. An integrating sphere has a 180° view of the reflected light, and can be used for convex curved surfaces, or to measure the color of objects that are small enough to fit into the sphere. Ocean Optics integrating spheres view a 6 mm or 8 mm spot size of the sample.
  
  An integrating sphere’s magic comes from its perfectly diffusing interior surface. Light enters through a circular input port and is scattered repeatedly by the sphere’s inner wall until the light inside the sphere is uniform, regardless of any spatial, angular, or polarization variations in the input. A fiber placed at 90° to the input port then samples a tiny fraction of the light within the sphere, sending it to the spectrometer. A baffle in front of the fiber port helps block any light rays making their first reflection from the sample port. 

• Collimating lenses. Add collimating lenses to optical fibers to customize the angle of incidence and angle of collection when making reflectance measurements. Specular or diffuse reflectance can be measured this way, but much more alignment is needed up front, as is extra fixturing. Also, the collimating lenses need to be adjusted carefully to avoid beam divergence and get good signal, making this a more time-consuming method. Finally, our experience is that color measurements made using collimating lenses and fibers, which are challenging to manipulate with precision, are not as accurate as those made using an integrating sphere.

Reflectance Standards

A reflectance system is not complete without a reference standard. Reflectance measurements are a ratio of the reflected light spectrum to the incident light spectrum. Since there is no way to collect all the light incident on a surface, reflectivity is usually measured relative to a reference standard.

The reflectance standard you select should be similar in reflectivity to the sample you’re measuring, to keep signal levels about the same during measurements and achieve the best signal-to-noise performance. Here are your options:

• Diffuse reflectance standards. The WS-1 diffuse reflectance standard has a PTFE diffuser, is matte white in color, and is >98% reflective from 250-1500 nm. The WS-1-SL has Spectralon diffusing material and can be a good choice when working in the field or in contaminated environments, since it can be smoothed, flattened and cleaned if it gets pitted or dirty.
  
  
• Specular reflectance standards. The STAN-SSH high reflectivity specular reference standard is the best choice when measuring very shiny surfaces, but it varies in reflectivity from 85%-98% over its 250-2500 nm range. This can be corrected in OceanView software; just upload the reflectance values and correct for the reflectivity of the standard. This data comes automatically with the STAN-SSH-NIST calibrated reference standard. If no correction is applied, OceanView will assume the standard is 100% reflective at all wavelengths, giving distorted data.
  
  At the other extreme, the STAN-SSL low reflectivity specular reflectance standard is best for surfaces with low specular reflectance values like thin film coatings, anti-reflective coatings, blocking filters and substrates. It has just ~4.0% reflectance from 200-2500 nm.

Here’s a sample reflectance system that uses a probe as the sampling device. The tablet shown here is a stand-in for laptop PCs and other computing devices. Also, this is just one example of dozens of different reflectance and color measurement configurations possible with Ocean Optics spectrometers and accessories.