If you can't see the product you can't measure it. Lighting is perhaps the most fundamental factor when measuring with Vision probes. It is also the FIRST parameter to enable when measuring an edge.
The type of lighting, the intensity and the mixture of lighting sources can have significant effect on the accuracy of your Vision system. Where possible, use only sub-stage lighting, as it will reduce the amount of texture on the surface and improve edge detection performance.
You can "Calibrate Illumination" and make needed adjustments via the "Probe Toolbox: Illumination tab" to ensure proper lighting for measurement.