Image Quality Factors

Low Light

Image Quality Factors
  1. Introduction
  2. Measuring low light
  3. Conclusion
  4. References

Introduction

Low light performance is defined as the lowest light level that a camera can capture an image while still retaining acceptable image quality. Cameras with small sensors such as mobile phones often encounter problems with low light performance. Measuring low light performance has, in the past, generally seen inconsistent or illogical test procedures (e.g., sensitivity of 0 lux). In 2018, however, ISO standard 19093 was published to outline measurement methods and specifications for the testing and analysis of low light performance.1 Low light testing has now become a critical aspect of overall camera quality evaluation.

Measuring low light

Before the standard was published in 2018, there were almost no universal test methods and the ones that did exist did not provide accurate evaluations. The test method described in the standard uses a multipurpose test chart.

This chart, the TE42-LL, was created by the TC42 working group in collaboration with the Image Engineering chart production team. The chart itself contains all of the important image quality testing structures for measuring the low light performance.

TE42-LL
Image 1: The TE42-LL test chart is designed for ISO 19093.

The first step of a low light evaluation is to ensure that the chart has proper uniform illumination. As low light testing naturally requires lower levels of light during the evaluation, it is important to use a light source that can be adjusted for various light levels. Currently, the iQ-Flatlight from Image Engineering is the only commercially available lighting system that can provide a wide light spectrum range at the required intensity levels.

Once the lighting is set, ISO 19093 then states that a reference image should be captured using bright light conditions at >1000 lux. From there the light level should continue to be reduced step by step with an image captured at each light level. The images are then evaluated for all of the necessary image quality aspects such as texture, color, noise, etc.

The lowest light level where all image quality factors are still above the required thresholds (for each of those factors) is reported as the low light performance of the camera under test.

low light levels
Image 2: Various light levels during low light testing.

The image quality factors that are determined are:

  • Exposure time to make sure that the image does not get blurry due to human handshake.
  • Exposure level as images are sometimes underexposed to maintain short exposure times and hide high amounts of noise.
  • Texture (low contrast fine detail) can be lost due to noise reduction applied to an image at high amplification levels.
  • Noise can occur due to high amplification levels.
  • Chroma decrease might be visible due to noise reduction in color channels only.

Table 1 (shown below) outlines an example of the results obtained from a low light performance measurement. The low light performance of this camera is 3 lux which is the last level where all values are green (above the thresholds).

low light value table
Table 1: Acceptable values of a partiular performance indicator are highlighted in green.

These thresholds are merely examples as ISO 19093 does not define a single set of thresholds. It only describes the procedure and users can set their own thresholds depending on the application of the camera. There are currently a few groups that have defined a set of thresholds for cameras for specific applications. For example, the VCX group for cellphone analysis is working on a set that they wish to establish for evaluating the low light capabilities of a cell phone.

Conclusion

The low light performance of a camera is defined as the lowest light level that a camera can capture while still retaining acceptable values across the various image quality factors. Until ISO standard 19093 was published in 2018, there was no other established guideline for measuring low light. ISO 19093 introduced a method that relies on a multipurpose test chart for analyzing low light performance. This test chart includes all of the necessary test targets (color, noise, resolution, etc.) for measuring the most important image quality factors.

Apart from the chart, the other most crucial part of analyzing low light is utilizing a light source that can be adjusted to various light levels (the iQ-Flatlight is currently the only light on the market with this ability). Together this chart and light source provide a proper solution for measuring the low light performance of a camera.