Gamma Scientific’s Thin Film Measurement Systems provide isolated first surface measurement of anti-reflection coatings on substrates down to 0.5 mm thick. Our unique technology allows manufacturers to perform single sided inspection on the first surface of any glass or polished, transmissive element while excluding the second surface.
The systems capture complete spectral and colorimetric properties of AR coatings with a scan time of less than 0.2 seconds.
Typical results from the measurement of antireflection thin film coating are shown in this section and are compared with measurements made with the second surface having an index matched liquid between the back surface and a piece of black absorption filter material.
Thin Film Reflectance Measurement Process
The first step in the measurement process is to calibrate the reflectance measurement scale of the spectroradiometer connected to the Model 191 Gonioreflectometer optical head for the chosen angle of incidence. This is accomplished by using the known physical properties of glass materials and the known physical properties of light interaction with materials. The glass material used as the standard reference to establish the scale of reflectance is Schott BK7 optical glass.
The spectral index of refraction of this glass from this manufacturer is known to a high precision and is held to a tight tolerance; BK-7 was chosen because of the negligible change in the index of refraction from melt to melt, +/- 0.001 for 546.1 and 587.6 nanometers and the index within a single melt varies less than +/- 0.0001 for 587.6 nanometers.
Using the known spectral index of refraction and the known characteristics of how light behaves at the intersection of two planar materials, each with a different index of refraction described by Snell’s Law and the Fresnel formula, the spectral reflectance of BK7 can be determined to a low uncertainty.
The reflectance of BK7 at a 30 degree incidence angle over a 360 to 940 nanometer range is shown below.
The scale for this plot is 0.042 to 0.0465 reflectance, equal to 4.2% to 4.65 % reflection.
Looking at the plot relative to a 1.0 reflectance or 100% reflection shows the magnitude of the BK7 reflectance relative to what a perfect mirror reflecting surface would give.
Now the BK7 is placed in the measurement plane of the gonioreflectometer optical head and used to standardize the measurement instrument.
The data below is the same BK7 sample used to calibrate, measured as an unknown. This step is taken to illustrate that a valid calibration has occurred. The difference between the calibration standard data and the measured data shown in this plot is 0.14% different in the visible portion of the spectrum.
Now the first surface of the coated sample, in this case an antireflection (AR) coating, can be placed in the measurement plane and the spectral data acquired as shown in the next section. This plot shows the anti-reflection coating with the uncoated BK7 data and the plot scale set to cover 0 to 0.15 reflectance or 0% to 15% reflection.
The logarithmic scale plot shown below gives a more defined image of the low reflectance regions of the anti-reflection coating.
Some anti-reflection coated products can have coating on both sides of the glass article and this second side coating can be easily measured by flipping over the sample and measuring the back side first surface. For this sample the back side spectral reflectance is shown below and the next graph shows it on a logarithmic scale plot.
Logarithmic scale plot of BK7, an anti-reflection coated glass plate and the coated back side of the glass plate reflectance.
Now with this data the spectral reflectance of both surfaces can be predicted to a first order by simply adding these two spectral reflectance data files of the front and back surfaces. The only difference between the sum and the measured reflectance of both surfaces is the spectral transmittance losses or fluorescence gains from light passing through the glass substrate material.
The resulting spectral reflectance obtained by adding the front and backside reflectances is shown below.
From this plot you see that there is only a small region of wavelengths between 640 and 700 nanometers that has a lower reflectance than the uncoated BK7 single first surface reflectance. The difference between both surfaces measured and the sum reflectance of the two individual surface measurements is shown in next plot and shows both surfaces measured reflectance lower than the sum of each surface reflectance; this difference is a result of the attenuation of the glass material.
If both surfaces measured reflectance function is divided by the summed each surface reflectance function the result is the spectral transmittance of the glass substrate which is shown below.
The next step involves the temporary contact of a black absorption filter glass such as Schott RG1000 to the back side of the coated substrate using an index of refraction matching liquid. This will simulate the use of the coated substrate in the final application.
In this condition the first surface alone was once again measured and both surfaces measured with the bonded black absorption filter material. The results of these measurements are shown in the plot below showing the advantage of the antireflection coating.
In this brief overview of the Gamma Scientific Model 191 Gonioreflectometer Spectroradiometer systems only a portion of all of the capabilities for material and process control are shown.
Thin Film Measurement Systems from Gamma Scientific
In the over 50 years of Gamma Scientific providing comprehensive optical radiation measurement systems and instruments, the spectral gonioreflectometer systems have been used to measure not only antireflection coatings for spectral efficiency but also other optical performance metrics such as:
- Color reproducibility
- Coating uniformity when coupled to an automated motion system; gantry or robotic arm
- Coating thickness
- High reflectance coating characterization
- Solar panel coatings
The customer set for these myriad of measurements include the world’s best companies whose focus on quality sets them apart from others. They include:
- Panasonic (Matsushita)
- Hewlett Packard
- National Renewable Energy Laboratory
- Sola Optical
- Optical Coating Laboratories
- Boeing Vertol
- Lockheed Martin
- Lawrence Livermore National Laboratory
- Fuji Film
- Arco Solar
- Carson Optical
- Volpi AG
- Northrup Grumman
- Rockwell Collins