Visual inspection using backlight testing (visual inspection)

It is often the case that test pieces have to be visually inspected and evaluated by employees under backlight. Our luminaires are specially adapted to the needs and applications of testing relating to such manual inspection tasks.

We will be happy to advise you on the right choice and necessary adaptation. Thanks to many years of experience, we pay attention to parameters such as light intensity, colour and quality as well as the size of the luminaire and how it can be adapted to the person in question.

Just ask for or Tel. +49 (0) 9364 8060-0).

Our lights as background light (back light)

Our Color Control product group consists of various systems that are equipped with a color rendering of approx. Ra95. This means that a good color assessment is always possible if required. The lights are always dimmable to adjust the brightness of the test task and the environment.

When developing the luminaire, we followed the following standards:

  • DIN EN ISO 3668 (visual comparison of the color of coating, D65)
  • DIN 3664 (viewing conditions for the graphics industry, D50)
  • DIN 6173 (color matching)

The three types of backlight tests

In general three ways of backlight inspection are distinguished. Firstly, translucent materials are transilluminated. Furthermore, shapes or breakouts of test objects are inspected. Also applied is a combination of backlight and oblique light, mainly when when glossy materials are checked.

Transillumination of materials

Here, translucent materials are illuminated with an adapted light source. The intention here is to detect the following:

  • Material inclusions
  • Purity (detection of foreign bodies)
  • Fill level monitoring
  • Control of sealing seams
  • General damage,


Brightness and light colour must be adapted to the respective test objects. The characteristics to be recognised must neither be too bright (outshined) nor too darkly illuminated.

Inspection of edges and borders

Here the contours of the test object are backlit by special test lights. To be checked:

  • Misplaced punching (outer edge and drill holes)
  • Roughness on edges
  • Material damage
  • Outbreaks,


The luminaire must not be too bright at this point either, otherwise the edges will be over-illuminated. The features would therefore no longer be visible.

Inspection of glossy surfaces

The luminaire is located opposite the observer (mixture of backlighting and incident light observation) and is reflected in the object. This creates a good contrast between the characteristics to be recognised. These include:

  • Flaws
  • Scratches
  • Fingerprints
  • Residues,


For this purpose, the luminaire must be selected in the correct size, otherwise the reflection and thus the test area is too small – especially in the case of cumbersome test objects. The right brightness is also decisive here. We recommend a dimmable version.

Selection criteria for transmitted light testing:

In order to make the correct selection of a luminaire for a visual transmitted light inspection, various criteria must be considered. We have compiled the most important ones.

The correct light intensity (brightness)

The correct light intensity (brightness)

LED backlight with proven technology for simple applications

One often hears “It can’t be bright enough”. But this is not correct. Depending on the test piece, the characteristic to be detected and the test environment, the luminaire must not be too dark, but also not too bright.

It’s all about finding the golden mean.

The inspector also plays a decisive role. The ideal brightness may vary depending on age or brightness perception. We always recommend a dimmable and therefore flexible variant.

The right light colour

The right light colour

Bright and robust LED backlights for continuous operation applications

It all depends on the correct light colour of the test luminaire. Usually a neutral white with about 5000–6000 K is chosen, as this is most similar to sunlight (neutral white). Our eyes perceive this light colour as accustomed and the human being reacts neutrally. If the light colour is too warm, this can lead to premature signs of fatigue in the inspector. A light colour that is too cold also appears to be unfamiliar or annoying.

The general light colour in the room should also be neutral white. When changing the viewing direction away from the test light, no accommodation of the eye is then necessary.

The light quality of the light source (Ra value)

The light quality of the light source (Ra value)

Slim LED light panel

Not only the light colour, but also the light quality of the light source is important. The Ra value indicates the composition of the light spectrum of the LED.  In general, only the Ri value is often used. For LED’s, the Rj value (extended Ri value) should really be specified. This contains the strong basic colours in addition. planistar always specifies both values.

It is important to have as high a value as possible in each R value:

„Only the light colours reflected by the light source on the test object can be perceived by the eye.“

The correct size and design of the luminaire

The correct size and design of the luminaire

very bright LED backlight for continuous as well as flash operation

Often, the test light is designed too small (not much larger than the test piece). However, the small test field is disadvantageous for the inspector. Since the employee’s actual field of vision is larger, the eye has to adapt to the contrast between the luminous surface and the darker surroundings when there are the slightest changes in the direction of vision. This has a negative effect on concentration and promotes fatigue.

Important: The backlight cannot be too large, but easily too small.

Customisation options for the inspector and the environment

Customisation options for the inspector and the environment

For a visual inspection, we always recommend the use of a dimmer at the workplace. The perception of brightness varies depending on the environment, time of day or employee. The concentration and fault detection can be increased by an individual adaptation of the test light.

Do you have any questions?

We would be pleased to advise you on your visual inspection task. To meet your needs, planistar has a wide selection of products in its standard range. The standard luminaire can be adapted to your requirements by simple modifications.

More information and points of contact:

+49 (0) 9364

We will be happy to develop tailor-made solutions for you.

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