How do I read a spectral plot correctly?

Reading a spectral plot is essential for evaluating thin film optical filters, such as bandpass, longpass, or notch filters. These plots show how much light the filter will transmit or block as a function of wavelength, so understanding their structure is key for choosing or designing the right filter.

1. Identify the Pass Bands and Stop Bands
The pass band appears as a region of high transmission (often >90%) over a specific wavelength range. The stop band is where transmission drops to near zero or to a very low value, indicating the wavelengths the filter blocks. Check the x-axis (wavelength, typically in nanometers) and the y-axis (usually percent transmission or optical density).

2. Examine Edge Steepness and Bandwidth
The steepness of the transition between pass and stop bands (the band edge) is important. A sharp edge means the filter can more precisely separate wavelengths. The bandwidth (often specified as Full Width at Half Maximum, FWHM) shows how wide the pass band is—does it match your application's requirements?

3. Check Blocking Level and Ripple
The blocking level (sometimes given as optical density, OD) defines how well unwanted wavelengths are suppressed in the stop band. Ripple refers to small oscillations in the pass band or stop band—ideally, these should be minimal for clean performance.

4. Angle Sensitivity and Practical Use
Most thin film filters are specified for normal (0°) incidence. If light hits the filter at an angle, the spectrum can shift toward shorter wavelengths (blue shift), and the pass band shape can distort. Always check the filter's recommended angle of incidence (AOI) and consider this in your optical design.

• Bandpass filter example: Transmission is high only in a narrow region (the pass band), nearly zero elsewhere. Useful for isolating a single color of light.
• Longpass/shortpass filter: High transmission above or below a cutoff, sharply dropping at the edge.
• Notch filter: One or more stop bands show as narrow dips in an otherwise high-transmission spectrum.

5. Measurement Tips

• Use a calibrated spectrophotometer for accurate results.
• Read both the x-axis and y-axis scales carefully when comparing filters.
• Check manufacturer's specifications and custom options if standard filters do not meet your needs.
• Mount filters at the recommended angle and orientation for best performance.

• Interpret pass band, stop band, bandwidth, and edge steepness carefully to assess filter quality.
• Consider angle sensitivity—using filters outside their specified AOI can cause unwanted spectral shifts and performance loss.
• Choose filters with sharp transitions, high blocking, and minimal ripple for demanding applications.