What is Signal to Noise Ratio of image intensifier?

Introduction to image intensifiers and SNR

Image intensifiers are electro-optical devices that amplify low light images and convert them into visible images that can be seen by the human eye. They work by converting photons entering the device into electrons, accelerating the electrons to strike a phosphor screen, which emits light that is amplified many times over the original level of light entering the device. This allows users to see images in very low light conditions that would otherwise be too dark to see with the naked eye.

 

One important performance parameter of image intensifiers is the signal to noise ratio (SNR). SNR compares the level of the desired signal to the level of background noise. It indicates how clear the amplified image is compared to the noise introduced by the amplification process. A higher SNR indicates a clearer image with less noise. Maximizing SNR is critical for obtaining high quality images from an image intensifier.

 

There are several factors that affect the SNR of an image intensifier:

 

Key factors influencing SNR

 

 

1. Photocathode sensitivity

The photocathode is the photoemissive surface inside the image intensifier tube that converts the incoming photons into electrons. The quantum efficiency of the photocathode determines how efficiently it can convert photons to electrons. Photocathodes made from materials with higher quantum efficiencies can extract more signal electrons for a given number of incoming photons. This improves the signal strength relative to noise, increasing SNR. Common photocathode materials like S20 and S25 have high quantum efficiencies of over 20%.

 

2. Microchannel plate gain

The microchannel plate (MCP) is a thin glass disc with microscopic channels coated on the inside with a semiconductive material. It multiplies the electrons emitted from the photocathode by a factor of 1000 or more to amplify the signal. The microchannel plate gain depends on the voltage applied across it. Higher gain produces more electrons to generate a stronger image signal, but also amplifies noise. Optimal gain levels around 1000 balance amplification with noise.

 

3. Phosphor screen efficiency

The amplified electrons exiting the MCP strike a phosphor screen, which converts the electrons back into photons to form the visible image. Phosphor screens made from materials like P20 have high conversion efficiencies, producing more light photons per electron. This increases the signal strength without amplifying additional noise.

 

 

4. Power supply voltage

The image intensifier components require specific voltage levels applied across them to function. The power supply provides these operating voltages. Higher voltages generally increase gain and signal strength, but also increase noise. Operating at optimal voltage levels improves SNR. Power supply ripple and fluctuations also introduce noise that can degrade SNR.

 

5. Light levels

Higher input light levels contain more signal photons that get amplified by the image intensifier. This increases the amplified signal relative to noise. However, image intensifiers are designed to work at very low light levels. Excessively bright lights can overload the intensifier and actually decrease SNR due to saturation effects. Proper light management maximizes SNR.

 

6. Operating temperature

The intensifier components are sensitive to temperature changes. Higher operating temperatures increase thermally generated dark current noise in the photocathode and MCP. Cooling the image intensifier housing reduces this dark current noise and improves SNR, especially in long wavelength (near-infrared) intensifiers.

 

7. Secondary electron emission

The semiconductive MCP channel coating has a characteristic secondary electron emission curve. Operating at the peak of this curve maximizes electron multiplication gain and signal amplification. Biasing below the peak decreases gain and SNR, while biasing beyond the peak introduces excess noise. Careful gain calibration is required.

 

8. MCP strip current

Too high a strip current through the MCP channels during operation can lead to gain saturation and increased noise. Limiting the strip current prevents charge depletion in the channels and optimizes gain and SNR performance. Strip current is managed by the voltage across and resistance of the MCP.

 

9. Image intensifier resolution

Higher resolution intensifiers with smaller pixel sizes preserve more spatial details in the image but also gather less light per pixel. This can reduce signal strength and SNR compared to lower resolution units. Pixel sizes around 25 microns offer a good balance between resolution and light collection.

 

10. MCP pore size

The MCP pores multiply the photoelectrons. Smaller pore sizes (10-12 microns) provide higher electron gains but also backscatter more electrons, increasing noise. Larger pore sizes (25 microns) have lower noise but also lower gain. 12-15 micron pores offer optimal gain versus noise.

 

Achieving high SNR in image intensifiers

Based on the factors above, the key steps to achieve a high SNR in an image intensifier system are:

 

Use a high quantum efficiency photocathode material like GaAs, InGaAs, or high QE S20/S25

Operate the MCP at optimal gain around 1000X

Use a high efficiency P20 phosphor screen

Power the image intensifier at recommended operating voltages

Manage light levels to maximize signal photons

Cool the image intensifier housing to reduce dark current noise

Carefully calibrate and operate the MCP at the peak of its SEM curve

Limit MCP strip current to prevent charge depletion

Use higher resolution intensifiers with pixel sizes around 25 microns

Select MCPs with pore sizes of 12-15 microns

With careful optimization of these parameters, high-performance generation 3 image intensifier tubes can achieve SNR levels of over 20 dB, providing outstanding image quality even in extremely low light conditions below 0.001 lux. Proper understanding and control of the factors influencing SNR is critical to maximizing the image quality obtainable from image intensifier systems.

 

Conclusion

In summary, the key parameters that determine the signal to noise ratio of an image intensifier are the efficiency of its photocathode and phosphor screen components, the gain and operating conditions of its microchannel plate, input light levels, operating temperature, secondary electron characteristics, power supply voltages, resolution, and pore size. Carefully optimizing all these factors enables modern image intensifiers to operate with very high SNRs that deliver outstanding image quality amplification even in the most challenging low-light conditions.

 

References

[1] Bangstad, T., Pust, N., & Petersen, A. (2019). The Technology of Image Intensifiers. In Photonics Technologies for Robot Vision Systems (pp. 107-144). Springer, Cham.

[2] Kuhl, J., & Leach, R. (2018). Quantum efficiency theory of image intensifier tubes. Optical Engineering, 57(5), 1.

[3] Design of image intensifier tubes for military night vision devices. https://www.semanticscholar.org/paper/Design-of-image-intensifier-tubes-for-military-Feng-Gong/2c3f363110609b1d25a3b81bf3b4a7604fb5417f

[4] Bangstad, T., Pust, N., & Petersen, A. (2019). Image Intensifier Tube Generations and Characteristics. In Photonics Technologies for Robot Vision Systems (pp. 71-105). Springer, Cham.

[5] Liang, E., Liu, X., & Liu, H. (2017). High-resolution low-light-level image intensifiers. National Science Review, 4(3), 422-432.

[6] Jiang, M., Liu, X., Liu, H., & Liang, E. (2020). High-performance image intensifier tube with 47-lp/mm resolution. Optics Express, 28(10), 14760-14772.