Optical film density in film radiography and grey value in CR and DR have some similarities but still they are not same.

When we increase the radiation exposure of film then optical density increases. Similarly increase in exposure results in increase in grey value in CR up to a certain level as determined by bit value. Film follows sensitometric curve which is not a straight line. CR software may have facility to select linear or logarithmic curve. Standard specifies the density range for example ASME Sec V specifies 2 to 4 density range for gamma radiography. But it does not specify required grey value range because by adjusting window-level we can view different range of grey values. Also different equipment/software follows different grey value range.

SNR does not related with optical density. SNR is expected to be high for better digital image quality.

Could you please specify what type of conversion chart you are looking for?

The key difference is that film density and CR grey value measure exposure in fundamentally different ways, so a simple conversion chart won't work across all systems.

Film radiography uses optical density (OD), a logarithmic scale where density increases predictably with exposure following the film's characteristic curve. ASME Sec V and other codes specify absolute density ranges—typically 2.0 to 4.0 for gamma radiography—because the film's response is consistent from batch to batch and lab to lab.

CR and DR systems capture the same exposure information but store it as digital grey values (typically 0–4095 for 12-bit systems, or 0–65535 for 16-bit). The grey value is then adjusted by window and level settings during review, so the same exposure can appear as any grey value depending on the operator's display choice. This is why standards don't specify grey value ranges—they're not fixed properties of the image.

SNR (signal-to-noise ratio) describes image quality, not exposure. Higher SNR means less noise in the image and better visibility of defects. In film radiography, SNR improves with proper exposure and development. In CR/DR, SNR improves with adequate dose, the sensor's bit depth, and the equipment's noise characteristics. Underexposure in either method degrades SNR and reduces your ability to detect small defects.

If you need to compare film and digital systems for acceptance, work with the specific code (ASME, API, EN, ISO) that governs your inspection. It will specify the density or image quality standard you need to meet, and your procedure should detail how to verify it on your chosen technology.