It could be some digital range that the camera uses, like it's its own proprietary scale. You'd have to ask the manufacturer to learn what those values mean, and most importantly, which value gives a linear signal with gamma of one. Usually you'll want the gamma to be one for image analysis based on intensity because that means twice the illumination level on the sensor gives twice the gray level. If the gamma is not one, like if it's 2, then doubling the light hitting the sensor will not double the gray levels out. However because higher gamma tend to suppress bright regions of the scene while brightening dark regions of the scene (enabling us to see more detail in dark regions), a higher gamma is often used to create images for pleasing images viewed qualitatively by humans (but again, not as useful for numerical/quantitative image analysis).