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Pros and Cons of Thermopile and Si Photodiode Sensors For Quantifying Broadband NUV Radiation

Broadband NUV light sources are the workhorses in lithography and UV curing applications. Quantifying their radiation accurately ensures repeatability in research labs and is essential for process stability both in low and high-volume manufacturing. Thermopiles and Si photodiodes are the most widely used sensor types for broadband NUV radiation measurement, each coming with its own set of advantages and disadvantages:

Silicon (Si) Photodiode Sensors


Fast Response Time: Si photodiode sensors have a fast response time, typically in the nano- to microseconds range. This allows for real-time measurements of rapidly changing radiation levels.

Compact Size: These sensors are available in compact sizes and various configurations, making them easy to integrate into different measurement setups and devices.

Low Power Consumption: Si photodiodes require minimal power to operate, making them ideal for battery-powered devices such as portable UV radiometers.


Limited Dynamic Range: Si photodiodes have a limited dynamic range compared to thermopile sensors. They quickly saturate, limiting the operating range with reasonable resolution to a relatively small predefined intensity band.

Temperature Sensitivity: Si photodiode performance can be affected by temperature fluctuations, potentially leading to inaccuracies in measurements if not properly compensated.

Spectral Sensitivity: The spectral response of Si photodiodes is non-uniform across the NUV wavelength range, requiring careful calibration and correction for accurate measurements.

Spectral Respose of Thermopile and Si Photodiode Sensors
Typical spectral response of thermopile and Si photodiode sensors

Thermopile Sensors


Wide Dynamic Range: Thermopile sensors offer a broader dynamic range than Si photodiodes. They can accurately measure a wide range of radiation intensities without saturation.

Flat Spectral Response: A flat spectral response minimizes variations in sensitivity to different wavelengths. This consistency is essential for obtaining accurate and reliable measurements, especially in situations where the spectral distribution of the light source is not well-controlled or known in advance.

Stability: Thermopile sensors tend to exhibit good long-term stability, ensuring consistent measurements over extended periods.


Slower Response Time: Thermopile sensors have a slow response time, typically in the seconds range. This makes them less suitable for capturing fast-changing radiation events with high temporal resolution.

Bulkier: Thermopile sensors are generally larger and bulkier than Si photodiode setups, limiting their potential integration into compact or space-constrained devices.

Cost: Thermopile sensors tend to be less cost-effective than Si photodiode sensors.

Our Advice

When it comes to quantifying NUV radiation, it’s important to understand the specific requirements of your application to determine the optimal sensor type. In considering your options, keep in mind that both thermopile sensors and Si photodiode sensors have unique advantages and disadvantages.

You can obtain very accurate NUV radiation measurements with Si photodiodes if you know the spectral distribution of the radiation and calibrate your sensor accordingly. In addition, Si photodiode sensors are basically the only option for high-speed light measurement applications (e.g., real-time monitoring of light output). On the other hand, thermopile sensors offer a wide dynamic range and a flat spectral response. In fact, their low spectral sensitivity makes them the perfect tool for equipment and process engineers to perform accurate NUV measurements in the field and to reliably set targeted broadband NUV intensity levels in lithography and light curing applications.

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