Density is a very important word, yet seems to be one of the most difficult to understand. I hope this will help clear things up.
Density: One of the photographic properties that comprise visibility of detail; the degree of overall blackening of the film.
Optimal density for human visibility is .25-2.5. We give the different shades from white to black a number between 0 and 3.
If I were to take the density reading of the bones in the hand the reading would be closer to zero. When looking at the bones we can see they are mostly white and have low density number.
Here’s the tricky part. The bones in the hand are the thickest, or most dense part of the hand. This makes it difficult for the x-rays to make it through to the film. Not having the x-ray photons reach the film, or by having an insignificant amount hitting the film makes it appear white.
The flesh, or soft tissue, of the hand is the least dense and therefore allows for the x-ray photons to pass easily to the film. Having many x-ray photons interact with the film causes the density on the film to black, or grey. The shades of grey fall between .25-2.5 with 2.5 being completely black to the human eye.
Knowing this we can say that the thickness of the body part is inversely proportional to the density on the film. The thicker the body part, the lower the density number.
As you train to become a radiographer you start to refine your ability to tell if a film is of optimal density or not. You will begin to notice the difference in the density of Chest x-ray which is for soft tissue, and a lumbar spine which is thick bone.
When a film is overexposed it has a high density number and has blackening on the film. An overexposed has too much information on it. Too many x-ray photons have made it to the film
When a film is underexposed it has a low density number and is white or lighter shades of grey. An underexposed film has not received enough information on it.
If too much information has made it to the film a bright light or “hot light” can be used, or better yet if you are using a digital imaging system the scanner can eliminate the excess information. When using a digital system you are able to adjust your window level to help rid of the extra information.
If your film is underexposed and not enough x-ray photons made it to your film, not enough information, you cannot adjust your image or use a light to make the image appear better.
Knowing this, if you were given the choice to use too much or too little it would make sense to use too much. Having said that, you still need to consider patient dose and not consistently overexpose your patients.
~Hope this helps!