Best practices for using transmissive test targets

January 18, 2017
June 26, 2017

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. We have performed a study of lightbox uniformity to see how different models compare. 

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 chart you need is based on the distance you want to test at. Test distance depends on whether you have a fixed focus or auto focus 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 illuminates an area greater than or equal to your imaging plane. Many charts we sell are sized for particular lightbox models such as SFRplus for GTI lightboxes, or the ultra high precision composite Film SFRplus chart

Masking

You may choose a lightbox that is larger than your imaging plane at the distance you are testing at, 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 light box, 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 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 construct an or 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 their unit under test facing upwards, with the lightbox & chart facing down. In this situation, adhesive tapes strips will have difficulty withstand the gravity of the chart, and will eventually de-adhere. A sheet of double-sided (possibly optically clear) adhesive can be used between chart and the diffuser panel lightbox will increase the longevity of such a setup,

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.

 

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