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A backlit graphic that looks right under workshop lighting often looks washed out the moment the LEDs come on. The image has not changed, but the way light reaches your eye has. Getting consistent colour on backlit film, backlit banner and roll-up displays means building a transmissive ICC profile instead of reusing the reflective one.

Why the colour shifts when the light comes on

When light passes through a semi-translucent medium, it exaggerates the optical density gaps between ink layers. Yellow and red desaturate faster than blue and black, which pulls the whole image warm and flat. A profile built from reflected measurements has no way to account for this.

Step 1: linearise first

Before any colour is measured, the printer's raw ink limit has to be set. Print a target chart on the actual media and read the individual C, M, Y and K channels to find the maximum ink load the coating absorbs before it pools or bleeds. Skipping this step means every later measurement is taken on an unstable base.

Step 2: measure in transmission

• Print an expanded target (an IT8.7/4 chart has over 1,600 patches) on the production media.
• Lay it on a D65-calibrated light box so the light source matches the standard.
• Read every patch with a spectrophotometer in transmissive mode, not reflective mode.

Step 3: build and validate the profile

Import the transmission data into profiling software, generate the .icc or .icm file, and load it into the RIP. Then verify with a test print: target a Delta-E (ΔE) below 1.5, which is the point where the eye stops seeing a difference between production runs.