Best practices for using transmissive test charts
- Henry Koren
Light Source
The most uniform light source available would be an integrating sphere, but these come with considerable size and expense. After considering how the light source uniformity impacts their results, most lab & manufacturing tests use LED or fluorescent lightboxes with flat-panel diffusers. Some customers choose a larger lightbox because that leads to greater uniformity within the central image plane area. Compare models of several different uniform light sources.
To perform tests with a transmissive chart setup, devices with built-in light sources will need to have that light source disabled.
Chart
The size of the chart you need is based on the distance you want to test at. Test distance depends on whether you have a fixed focus or autofocus lens. You may select a single test distance or a range of distances between the minimum focus and hyperfocal distance, which may include macro and long-range resolution tests.
You will want to determine what the size of your imaging plane will be at your test distance(s). The Imatest Chart Finder utility can assist you in determining this. Unless you are dealing with an extreme wide-angle lens, you would typically want to choose a light source sized to illuminate an area greater than or equal to your imaging plane. The transmissive charts we sell are sized to fit the variety of lightboxes we sell and are printed on DisplayTrans with an inkjet process or the ultra-high precision film.
Masking
You may choose a lightbox that is larger than your imaging plane at the distance you are testing, or larger than the test chart that you are affixing to it.
Many transmissive test charts have a “base density” which causes light areas of the target to block significant amounts of light compared to a naked lightbox. Depending on the chart material this can range between 0.1 – 0.3 density units. If your test chart doesn’t fill your lightbox, you will be left with significantly brighter areas outside the test chart. This flood of bright light will disrupt your test by causing stray light (flare) that can impact your results by reducing contrast. This is especially for any dynamic range measurements.
In some cases where the flare is not a serious concern, you may have additional translucent material added to the sides of your test chart so that it fully covers your lightbox.
In other cases, you may purchase an opaque mask using matte black acrylic or other opaque cards which can block any additional light from reaching your lens.
Mounting
Test charts can be taped to a mask, or taped directly to the lightbox diffuser panel using double-sided tape. Since many common adhesive-backed tapes include acid that may react with film substrate, an archival tape is recommended.
Inkjet tests should have the glossy side of the chart facing towards the lightbox, with the matte surface with the pigments facing outwards.
Film and chrome on glass charts have a directional nature to them as well. These should be oriented with the chrome or film emulsion layer facing out towards the camera under test. This is typically confirmed by verifying that the chart model number text is not mirrored.
Some customers have the unit they are testing facing upwards, with the lightbox and chart facing downward. In this situation, a taped chart may not withstand the gravity over time and will de-adhere. A chart mounted to clear acrylic will not have this problem and is the better option in such a setup.
Transmissive charts you buy from Imatest will come pre-mounted on a clear acrylic for ease of use and accuracy of measurements. Mounting on acrylic eliminates the possibility of bending or bowing in the chart which can produce inaccurate measurements in Imatest software.
Environment
Glossy test charts are prone to specular reflections when there is ambient light within the test environment. With the exception of inkjet produced targets, most high precision and high dynamic range transmissive test targets have glossy surfaces.
Reflections can disrupt the accuracy of measurements, especially if they occur within critical test zones such as the dark areas of a dynamic range test chart. Dynamic range testing requires an environment where all additional light is blocked from the test area. Complete light-proof enclosures made of low reflectance materials such as dark opaque plastics, black felt, velvet, or anodized aluminum can enable a light-free environment that will enable accurate measurements.