Exposure Index
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Exposure Index
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Exposure index (EI) is the measure of the amount of exposure received by the image receptor (IR). It is dependent on mAs, total detector area irradiated, and beam attenuation. The exposure index is indicative of the image quality. Equipment manufacturers provide a recommended EI range for optimal image quality (Bontrager & Lampignano, 2005, p. 52).EI in digital radiography can be compared to film speed and blackening in film-screen. When film was used, the accuracy of the exposure was obvious based on the appearance of the image. Digital systems post-process images and display adequate contrast and brightness at a much wider range. Therefore, adequate exposure can only be assessed through image noise or burn-out. Secondary workstations such as those used by technologists for image review, are often of lower resolution and brightness than those used for diagnosis. Because of this, it is often difficult to assess whether an image is noisy or not. The exposure index is meant to be an indication of whether the noise levels are acceptable (AAPM, 2009).
Errors in the calculation can occur resulting in an inaccurate EI. This can arise when the software fails in determining which part of the image is the patient anatomy, for example, in the presence of gonadal shielding or prosthesis. EI cannot be solely relied on, therefore the technologist must remain critical of the appearance of the image and the accuracy of the EI (AAPM, 2009). EI is derived from the mean detector entrance exposure which is derived from the mean pixel value of the image. Most systems use a histogram analysis in order to calculate the mean pixel value (Neitzel, 2004, p. S231). 
This is a histogram created from an AP pelvis radiograph. The x-axis represents the pixel value while the y-axis represents the number of pixels with that value. The mean pixel value here is 104.381.
Although EI is always derived from the IR exposure, equipment manufacturers calculate the numeric value differently, resulting in different ranges and definitions (Carlton & Adler, 2006, p. 367; Neitzel, 2004, p. S231). Also, there is variation between units purchased from the same manufacturer based on different IRs and software (Carlton & Adler, 2006, p. 367). Different IRs have different detective quantum efficiency (DQE). A high DQE results in lower noise levels (AAPM, 2009, p. 3). Therefore, all systems have a different index and are difficult to compare across systems. Fuji CR Fuji uses a sensitivity number (S) that is related to the amount of amplification required by the photomultiplier tube to adjust the digital image. S is inversely proportional to exposure. Properly exposed images should have an S between 150-250 (Carlton & Adler, 2006, p. 367). Kodak CR Kodak uses the term Exposure Index, which is directly proportional to exposure. Properly exposed images should have an EI between 1,800-2,200 (Carlton & Adler, 2006, p. 367). A change of 300 in the EI indicates a change of a factor of 2 in the exposure to the IR. Agfa CR Agfa uses log median exposure (LgM). This system compares the exposure level of the image to a baseline established for the department. Since it is based on a log system, an increase of 0.3 means the dose was doubled (Carlton & Adler, 2006, p. 367). An optimal exposure lies between 1.9 and 2.5.
Imaging Dynamics uses f#. The f# compares the exposure to an established target exposure. Negative values represent underexposure, while positive values indicate overexposure (AAPM, 2009). Canon DR Canon uses a reached exposure value (REX). REX is a function of the brightness and contrast as selected by the operator (AAPM, 2009). GE DR GE uses the detector exposure index (DEI) which compares the detector exposure to the expected exposure value (AAPM, 2009). Siemens Future Developments In 2008, the International Electrotechnical Commission (IEC) developed and published the International Standard IEC 62494-1 on the definition and scaling of the exposure index for digital radiography. According to the standard the EI shall be proportional to the exposure (air kerma) and shall be scaled as EI = 100 * X, where X is the air kerma at the detector, at the calibration beam quality. It is expected that this standard definition will be implemented in future digital radiography systems. The American Association of Physicists in Medicine (2009), published a document in July, 2009 with the purpose of identifying a standard index which reflects the adequacy of the exposure received by the IR. References American Association of Physicists in Medicine. (2009). An Exposure Indicator for Digital Radiography. Retrieved from http://www.aapm.org/pubs/reports/rpt_116.pdf Bontrager, K. L., & Lampignano, J. P. (2005). Textbook of radiographic positioning and related anatomy (6th ed.). Elsevier Science. Carlton, R. R. & Adler, A. M. (2005). Principles of radiographic imaging: An art and a science. Delmar Learning. International Electrotechnical Commission (2008). IEC 62494-1 ed. 1 Medical electrical equipment – Exposure index of digital x-ray imaging systems – Part 1: Definitions and requirements for general radiography Neitzel, U. (2004). Management of pediatric radiation dose using Philips digital radiography. Pediatric Radiology, 34(Suppl 3), S227-S233. |
Exposure Index
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The exposure index (EI) of a film speed is the rating of a particular film that varies from the recommended ISO film speed. For instance, 100 film speed may actually be used as a 400 film speed in what is called pushing film. Conversely, a 400 speed film may be used as a 100 speed film in what is called pulling film. By changing the ISO film speed to the EI speed, the photographer is speeding up, or slowing down, a particular film. When pushing film, the photographer can gain an extra one or two stops to obtain good negatives in low-light or motion conditions. For instance, if a photographer rates an ISO 100 film at EI 200, they gain one stop in exposure. Read more about this in pushing or pulling film.
The EI film speed setting may also be used in cases where the shutter of a camera is off timing and always overexposes or underexposes the film. There may also be instance where the light meter is inaccurately reading. In these cases, the photographer may adjust the EI rating to compensate for these errors. By forcing the EI rating faster than the ISO speed (pushing film) the photographer is underexposing the film. Forcing the EI rating slower than the ISO speeding (pulling film) causes the film to be overexposed. It should also be noted that when a photographer changes the EI rating, the film will require a different development time to properly develop the negatives. In addition, you cannot change the ISO speed midway through a role or you will under/overexpose half the roll and will have no way of fixing this.
Why Adjust the EI Film Speed?
Accidents happen. If by chance you load 100 ISO film into your film camera and forget to adjust the ISO setting and leave it set to 400, you can still continue to shoot film and save it.
Don’t have the right film. There may be many cases where you need a faster film but are only equipped with slower films. You can push the slow film to a faster EI speed to improve exposures indoors or help with motion photography. There may also be instances where you want to slow the film to capture blur motion or other effects.
Contrast. In high contrast areas where there is an abundance of shade and bright light, lowering the film speed can help to reduce the dramatic difference in contrast. Conversely, increasing the film speed can add contrast.
Grain. This is not for everyone, but pushing film will help achieve grain that is associated with faster films. Some photographers like the look.
You must also understand that pushing or pulling film with different EI film speeds will require you to compensate when developing the film. Pushing the speed will require over-developing the film while pulling the speed will require under-developing it. Read more about EI processing in the push and pull processing page under the film processing section.
via Exposure Index – EI Film Rating | Gudie to Film Photography.
Weston Model 854 DR
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  Maker: Weston Electrical Instrument Model: Model 854 “DR” Circa: 1958 Cell type: Selenium Measure type:Reflecting/Averaging Designer: Alexander Williams US Patent: 2,667,809 |
.jpg) Weston’s last attempt at a simple, “direct reading” meter for the amateur niche: i.e. you just point it and read the exposure and that’s it. It actually has a good idea behind it: you use a revolving plate on the back that partially covers the selenium cell, and that calibrates the meter for the film speed (a faster speed uncovers more of the cell, so the reading is higher. Neat idea). There’s a knob on the side, and you turn the knob until your desired shutter speed shows up, and then direct read the aperture. Or turn the knob until you find an aperture you like, and read the resulting shutter speed.This is one of the most unusually styled meters that I own. Like other Westons, the cell is on the back and it was meant to be held upright at eye-level. So you hold the bottom portion (you can see the grip contours on the sides) and your thumb can roll the knob, making it one-handed. The truly neat thing is that the roll knob is on both sides, so you can use it in either hand. Most meters are laid out for right-handed use.
Despite the neat styling and features, and that fact that it is easy to use, it suffers from the same problem all direct-read meters have—by being simple, it’s severely limited. On this one, the E.I. settings only go to ASA 250, but high speed films rated at ASA 400, or pushed to E.I. 500 or higher, were becoming common. |
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