The core of the transmittance uniformity tester is to collect transmittance data from different areas of the sample at multiple points, and then determine uniformity by calculating and analyzing the differences. Its working principle can be broken down into four key steps: light source control, optical path conduction, signal detection, and data processing, as follows:
1、 Light source: Provides stable and controllable incident light
The light source system of the tester needs to meet the requirements of spectral stability and constant intensity to ensure consistent incident light parameters and avoid measurement accuracy being affected by light source fluctuations.
Common light sources include white LED, xenon lamp or tungsten halogen lamp. Some high-end instruments are equipped with monochromators, which can output monochromatic light of specific wavelengths (such as visible light 550nm, ultraviolet 365nm) to meet the testing needs of different materials (such as glass, film).
The light source is subjected to uniform light treatment through a lens or integrating sphere, so that the intensity distribution of the light incident on the sample surface is uniform, reducing the interference of the non-uniformity of the light source itself on the measurement.
2、 Optical path: Accurately transmit light and locate measurement points
The core of optical path design is to allow light to enter different areas of the sample vertically and ensure that reflected and stray light are effectively eliminated.
Mechanical positioning system: The sample stage can usually be driven by a motor to achieve X/Y axis movement, or an array type probe distribution can be used to automatically locate measurement points at preset intervals (such as 5mm × 5mm, 10mm × 10mm) (common points are 9 points, 16 points, or custom grids).
Shading and collimation: A shading hood and collimation lens are added to the optical path to make the incident light a parallel beam, avoiding overlapping measurement areas caused by light divergence, and ensuring that the light at each measurement point only comes from a specific small area of the sample (usually with a diameter of 1-3mm).
3、 Detection: Capture transmitted light and convert it into an electrical signal
After passing through the sample, the light is received by a photodetector and converted into a quantifiable electrical signal, which is the core process of measurement.
Detector type: commonly used silicon photodiodes or photomultiplier tubes, whose sensitivity matches the spectrum of the light source (such as silicon detectors that respond well to visible light), and can convert the transmitted light intensity into corresponding current or voltage signals.
Signal amplification and filtering: Weak electrical signals are amplified by an amplifier and filtered through a filtering circuit to remove interference such as ambient light and circuit noise, ensuring the stability and accuracy of the signal.
4、 Data processing: Calculate transmittance and analyze uniformity
By comparing the intensity of incident light and transmitted light, calculate the transmittance of each point, and then obtain the uniformity index through statistical analysis.
Transmittance calculation: The transmittance of a certain point T=(transmitted light intensity I ₜ/incident light intensity I ₀) × 100%, where I ₀ needs to be pre calibrated (i.e. "zeroed" or "baseline calibrated") when there is no sample.
Uniformity analysis:
Calculate the maximum (Tmax), minimum (Tmin), and average (Tavg) transmittance of all measuring points.
Common uniformity indicators include:
Deviation rate: (Tmax Tmin)/Tavg × 100% (reflecting the maximum difference);
Standard deviation: reflects the degree of dispersion between all measurement points and the mean, with smaller values indicating better uniformity.
Result output: The instrument outputs the transmittance values, distribution heatmaps, or uniformity levels (such as qualified/unqualified) of each point through the display screen or software, visually presenting the uniformity status of the sample.