Image Quality Factors

Flare

Image Quality Factors
  1. Introduction
    1. Veiling flare vs. image flare
  2. How does flare occur?
  3. How to measure veiling flare?
    1. Point spread function
    2. Using an integrating sphere for infinite measurements
    3. Chart based method for non-infinite measurements
    4. How to measure image flare using ISO 18844?
  4. Conclusion
  5. References

Introduction

Flare (also known as glare or veiling flare/glare) by definition is unwanted light in an image that did not originate from the related scene object. Flare can occur from multiple sources and when present, can greatly disrupt the quality of an image.

Veiling flare vs. image flare

Both veiling flare and image flare can have a negative impact on the final image. For example, veiling flare, according to ISO 93581, describes flare from a traditional camera lens. It is measured for just the lens removed from the camera. ISO 9358 outlines the test methods and procedures for analyzing veiling flare.

Image flare, meanwhile, describes the flare that occurs in the final processed image of a camera such as a mobile device. In many cases, the lens cannot be detached from the camera and it is unknown if a certain amount of flare originates from the lens, the camera, or even the image processing in the camera. As a result, ISO 188442 was created to outline a systematic approach to evaluating image flare for all types of cameras.

How does flare occur?

Flare can occur from many different variables including those from natural causes and those from the hardware components of the camera system.

Sources for flare include:

  • Everything between the object and the front lens of the camera, e.g., haze or dust in the air.
  • Interreflections between the optical parts of the lens and dust and dirt on the lens elements.
  • Light reflected from the edges and framing of the lens or the shutter, diaphragm, camera body, etc.
  • Light from the optical system itself with spherical aberration and chromatic aberrations etc.
  • Light leakage

Such an expansive range of variables, flare is often one of the more difficult image quality factors to measure.

Causes of flare
Image 1: The dark areas are experiencing a local increase in the signal is visible due to flare.
Causes of flare
Image 2: There should only be a bright spot on the facade of the One World Trade Center, but instead there are multiple dust particles in the image due to interreflections within the lens or sensor.

How to measure veiling flare?

ISO 9358 is the basic standard for measuring the flare of lenses. There are four different ways to measure veiling flare using ISO 9358 and they are:

  • Point spread function
  • Using an integrating sphere for infinite measurements
  • Using two integrating spheres for lenses with a large focal length
  • Chart based measurements for non-infinite measurements

Point spread function

The point spread function is measured by shining a small point light source onto the camera. The camera is then repeatedly rotated so that the light shines on the camera from all different angles. The resulting images are evaluated for flare, but there is currently no method to get a single number indicating the amount of flare present in these images using this measurement method.

Point spread
Figure 1: The principle for the determination of flare using a point light source.

Using an integrating sphere for infinite measurements

The second method mentioned in ISO 9358 uses a light trap in an integrating sphere. The black level measured in the area of the light trap is an indication of the amount of flare present. Two integrating spheres should be combined if there is a large focal length. ISO 9358 defines the infinite distance as a distance larger than 10 times the focal length. The diameter of the integrating sphere (the distance between the lens and the light trap) therefore needs to be at least 10 times the focal length.

Integrating sphere
Figure 2: Picture height distortion with barrel shape (negative)
two integrating spheres
Figure 3: Combine two integrating spheres for long focal lengths.

Chart based method for non-infinite measurements

A test chart can be used if the flare does not need to be measured at infinite distances. ISO 9358 uses a transparent test chart in its test setup as seen in figure 4.

transparent chart
Figure 4: The non-infinite method described in ISO 9358 uses a transparent test chart.

How to measure image flare using ISO 18844?

ISO 18844 is the best standard to use when analyzing the flare of a mobile phone (or similar) camera system. Unlike ISO 9358, which has multiple methods for measuring flare, ISO 18844 measures image flare using specifically designed test charts that are very similar to the charts from the non-infinite method of ISO 9358.

These test charts are uniform white targets with at least one black hole for determining the amount of flare added to the image by the camera. Put simply, the black hole acts as a light trap that absorbs a high proportion of the light thereby creating the lowest possible level of black. This level of black will rise if there is flare present in the image. This test chart can be either reflective or transmissive.

reflective chart
Figure 5: The use of a reflective test chart according to ISO 18844.
transmissive chart
Figure 6: Measuring flare using a transmissive chart according to ISO 18844.
flare chart
Image 3: Test chart TE281 is the flare chart that we prefer to use when measuring flare according to ISO 18844.

The chart shown below is specifically designed around the description given in ISO 18844. It uses 17 light traps to capture the maximum light possible while also providing multiple measurement points for the analysis. The contrast reaching more than 10.000:1 is sufficient when using the simplified method C as described in ISO 18844. From a single image, the ratio between the black and white levels can be obtained to indicate the amount of flare.

flare test results
Figure 7: A sample result using a Canon lens in front of a DSLR.

This method, as described in ISO 18844, currently does not take into account other light and light sources that are located outside the cameras field of view even when these light sources are often relevant to flare.

At this time, the responsible ISO group, as well as Image Engineering, are looking into various options to solve this problem.

Conclusion

Flare can be adequately analyzed using the methods described in either ISO 9358 or 18844. ISO 18844, however, has been specifically developed for all types of camera systems including those without the possibility to detach the lens (a requirement for ISO 9358).

When analyzing flare in our test lab, we almost always follow ISO 18844 as it is the most recent version and encompasses a greater number of camera systems including, most importantly, mobile phone cameras.

Using as test chart designed for flare (as described above) combined with proper illumination has consistently proven to provide the best results when analyzing where a camera system can be improved for flare.