Radiography in Modern Industry - Kodak

Since this formula applies only to light-transmitting images, it cannot be applied to an opaquebasedimaging material such as radiographic paper. Therefore, a slightly different means ofmeasuring density is necessary, and this is called reflection density, D R . Reflection density isdefined as the logarithm of the ratio of incident light intensity, I O , to the reflected light intensity, I Rfrom the image area. This formula is:So, although the formula appears to be quite similar to that of transmission density, in practicalapplication, reflection density measures the light reflected from the radiograph, not that whichpasses through. For example, reflection densities are measured by a reflection densitometer, andthe familiar densitometer for measuring transmitted densities cannot be used.To carry the discussion one step further, exposure is defined as the product of the quantity ofradiation--measured in roentgens or other units--and time. In this respect, the exposure toradiographic paper is measured exactly the same as it is for x-ray film, although the order ofmagnitude of the exposure may be different. Reflection characteristic curves can be generated forradiographic paper. The difference between these curves and those for film is that transmissiondensity values are used for x-ray film, while reflection densities are used for radiographic paper.Characteristic curves (which are also known as H & D curves) for paper have a shorter range ofdensities and usually a shorter log exposure scale.Comparable Densities--Paper and FilmInterpreters could easily be led astray at this point by becoming involved in the purely objectiverelationship between reflection density and transmission density. They may theorize, for example,that under a given set of viewing conditions, reflection density of 0.7 appears to them to be similarto a transmission density of 2.0. In reality, the transmission density to reflection densityrelationship has no bearing upon where the same radiographic information is recorded on thefilm's transmission density scale as related to the paper's reflection density scale. The questionreally is--which densities contain the same information if x-ray film and radiographic paper areused to record the same image?Given the correct intensity of illumination, it is universally believed that the most usefulinformation is recorded on the essentially straight line portion of the characteristic curve. In fact, itis rather generally accepted that in industrial radiography the higher the density of a filmradiograph, the better the visibility of tiny discontinuities limited mainly by the available intensity ofthe illuminator. Because of the essentially opaque base of paper radiographs and the shouldereffect of the H & D curve of paper, this concept cannot be applied to reflection densities.A radiographic image on paper of the same image area of a subject will contain the sameimportant image details as a radiograph on film. These details will be modified in density (andpossibly in contrast) because the response is fundamentally different.It must also be recognized, of course, that the total range of information capable of beingrecorded will be less on a paper radiograph because the reflection density scale is shorter than itsx-ray film counterpart. For example, a reflection density of 2.0 on a paper radiograph is so blackthat detail is completely obscured.In addition, because of the opaque nature of the paper base, the method of viewing reflecteddensities of images on paper is fundamentally different from the method for viewing transmittedRadiography in Modern Industry 179