Coated Printed Circuit Board Reflectance with the R400-7-VIS-NIR Reflection Probe
Black coatings are applied to a surface to minimize the reflection of light from the surface. In the case of an optical bench, the mechanical housing is painted black to reduce stray light in the spectrometer. With a range of black coatings available with varying reflectivity characteristics, reflection measurements can be used to assess the reflection properties from a coated surface. In this application note, we measure diffuse, specular and total reflection from printed circuit boards painted with different black coatings to assess the reflection properties of each coating.
In theory, a black surface should absorb all wavelengths of light. In reality, there are many shades of black with absorbance and reflectance properties that vary as a function of wavelength. In the case of optical coatings, the need to accurately characterize reflectance properties as a function of wavelength is critical to the design of an optical instrument. Any light that is not absorbed by the optical bench coating becomes stray light that has the potential to reach the detector and impact instrument performance. Since the detector cannot distinguish between light transmitted through the sample and stray light created in the optical bench, the presence of stray light limits the maximum absorbance level achievable. As the stray light level increases, the absorbance spectrum flattens until the maximum absorbance value is reached and no further absorption is detected. To improve instrument performance and enable the measurement of higher maximum absorbance levels, stray light is minimized through the use of absorptive black coatings. In addition, optical design elements like low stray light optics and physical baffles that trap stray light are used to reduce stray light and increase the maximum absorbance levels and performance of an instrument.
Since most black coatings are not entirely black, reflection measurements are made to characterize the reflection properties of a surface. The goal of these measurements is to identify coatings that minimize reflection in the wavelength range of interest. Reflection measurements can be made using a reflection probe like the R400-7-VIS-NIR 400 micron reflection probe.
A reflection probe collects light from the same angle at which the sample was illuminated, allowing it to be used for either specular or diffuse reflection measurements. It consists of 6 illumination fibers around a single read fiber, with a 25° full angle field of view. Although it may be tempting to connect the 6 fiber leg to the spectrometer, it is actually more efficient to mate the single fiber to the spectrometer. As shown in Figure 1, each illumination fiber projects a cone of light from the source, all of which overlap at the sample in the center, which is exactly where the read fiber is “looking.”
To measure specular reflectance, orient the probe at 90° to the sample in the reflection probe holder (RPH-1). For diffuse reflectance, use the 45° port. The reflection probe needs to be pulled back slightly from the surface of the sample within the probe holder, as the rays from the illumination fibers need some space in which to overlap and generate reflected light. The diameter of the sample area seen will be about ½ of the distance between the probe tip and the sample (i.e., working at 4.0 mm distance will give you a ~2.0 mm spot size). Using a reflection probe holder (RPH-1) will ensure a consistent working distance from one sample to the next (and when taking a reference measurement). Its matte black finish also helps to reduce ambient light.
When taking a dark measurement with a reflection probe, it is best to block the light at the lamp if possible. Turning the lamp off and then on again will disturb the lamp’s thermal equilibrium, requiring additional warm-up time and a new reference measurement. If this is not possible, it is also acceptable to point the lit probe into a dark space to take the dark measurement, provided there is minimal opportunity to scatter light back into the probe. Resist the temptation to point the probe at something black (like a piece of paper or a cover cloth), as objects that appear to absorb all wavelengths can actually reflect some colors preferentially. The goal is to trap all of the light exiting the probe.
The most important factors in choosing the right reflection probe for your measurement are wavelength range and amount of light needed. System sensitivity should be optimized for the reference standard, as samples are almost always less reflective. Choose your fiber size based on the sensitivity needed, not the spot size desired. The spot size is dominated by the working distance of the probe from the sample, and is easily varied. Remember, all fibers have the same 25° full angle field of view, so a 600 micron fiber has a spot size only 400 microns (0.4 mm) larger than a 200 micron fiber at the same working distance.
Probes with reference legs are available when the lamp needs to be monitored continuously. Additionally, standard or custom expanded wavelength (“mixed”) probes offer multiple lamp input legs and/or measurement legs to allow mixing of different wavelengths and output to different spectrometers for optimization in each wavelength range. For those looking at powders or dense solutions, an angled probe tipped with a 30° window permits the user to immerse the probe directly into the sample and still achieve a consistent working distance.
The reflection spectra for PCB’s coated with three types of black paint (flat black, glossy black and black chalkboard paint) were measured using a reflection probe. The PCB samples are shown on the lower left in the photo of the reflection probe setup shown in Figure 2. The equipment used for these measurements was a USB4000-VIS-NIR spectrometer (Grating #2, SLIT-25), R400-7-VIS-NIR reflection probe and HL-2000 tungsten halogen lamp. Due to the relatively low reflectivity of these samples, a gray standard was used (~20% diffuse reflectivity). Integration times ranged from 20-120 milliseconds with a boxcar setting of 5 and 20 scans to average. As shown in Figure 2, a ring stand was used to support the RPH-1 reflection probe holder for the measurements with a black hood used to shield the measurements from direct illumination by ambient light. Measurements were made with the probe arranged at 45° (diffuse reflectance) and 90° (specular reflectance) relative to the coated surface using the RPH-1 reflection probe holder.
Although measurements with a reflection probe may seem quite straightforward to acquire, the data may turn up some unexpected surprises. To demonstrate, we looked at three types of black paint: flat, glossy and “chalkboard.” As shown in Figure 3, the specular reflectance data shows the chalkboard paint to be least reflective, with the glossy black paint ~10x higher. The scale may look a little odd (250% reflectivity for the glossy black paint) until we remember that the reference defined as “100%” is actually a 20% diffuse reflectance standard.
Looking at the diffuse reflectivity data shown in Figure 4, we see that the flat and glossy black paints actually look quite similar in the visible range, which means they would appear to be very similar in color to the eye, even if one reflects significantly more light. The paint samples start to differ in the NIR, which indicates that whatever gives the paint its gloss may be an NIR reflective compound. What is surprising is that the chalkboard paint appears to have a higher diffuse reflectivity than the other black paints. The reflection probe isn’t quite showing the whole picture, however, just the diffuse reflectivity when illuminated and viewed at 45°.
In this example reflectance application, an R400-7-VIS-NIR reflection probe was used to measure the diffuse and specular reflectance properties of black coatings on printed circuit boards. The spectral data resulting from these relatively straightforward measurements could be used to select the optimal optical bench coating to reduce stray light and enhance instrument performance.
With the modular spectroscopy tools available, a range of additional measurements are possible to further characterize these samples ensuring the selection of the best coating possible. An integrating sphere like the ISP-REF integrating sphere with a built in tungsten halogen light source could be used to measure light scattered at 180 degrees from the sample surface enabling more accurate and absolute measurements. The ISP-R series of integrating spheres for reflectance with sphere diameters ranging from 30-80 mm in diameter are also available. Spectrometers, light sources and reflection probes covering different wavelengths ranges could be combined with reflectance standards and alternative sampling accessories to further extend the measurements. All of these components can be easily swapped to enable full reflectance characterization of sample reflectance.