The same factors that affect high density affectlow density.PLOTTING SPEED POINT ONA PROCESS CONTROL CHARTThe speed point (SP) is a measure of the effectivefilm speed or exposure index of a film. The speedpoint is determined by sensitometric tests. The speedpoint is established using a step on a sensi-strip witha density of 0.10 above gross fog for ground pictorialfilm. The speed point of aerial film is established byusing the step on a sensi-strip that has a density of0.30 above gross fog.Once the speed-point step is determined, that stepis read in successive sensi-strips and plotted on thecontrol chart. Neither effective film speed nor theISO for ground pictorial film should be confused witheffective aerial film speed because they are notequivalent.PLOTTING GROSS FOG ONA PROCESS CONTROL CHARTGross fog (B+F) is read from a "clear" area of acontrol strip; that is, an area that does not receiveexposure. All films have a gross fog density,resulting from several factors that may include thefollowing:The density of the film baseChemical fogAge fogThe development of unexposed silver halidesInadequate fixation (film not cleared)As stated earlier, the amount of information youuse to monitor or control your process depends onseveral factors. However, when you choose tomonitor more than one processing variable, youshould construct the appropriate control chart or usea piece of graph paper that can be posted near theprocess. Figure 2-14 shows a typical family ofcontrol charts for a process. A family of controlcharts, such as this, will provide you with a wealth ofinformation about the process. Also, all theinformation is in one place.LIMIT LINESThe upper- and lower-limit lines on a control chartare based on the assumption that the plotted points arerepresentative of a normal "population" or set ofcircumstances of the process. The limit lines,therefore, should include between them, all pointsrepresenting an unchanged or normal process. Limitlines can never be placed in such a manner that alldata are included between them; there will always bedeviations. Samples from a black-and-white process,for example, show a gamma average of 0.70. On asubsequent test, a sensitometric strip was found tohave a gamma of 0.80. Obviously this processappears to have changed or is changing. Should theprocess be altered? The answer must consider thefactor of probability.Two risks are involved in judging whether normallimits are exceeded. One risk occurs when a certainsampling appears outside one of the limit lines,indicating that the process is out of control, but theprocess is actually behaving normally and has notchanged. This situation is known as the alpha risk.The reverse is also possible; it appears that the processis normal when actually it has changed or is changing.This is called a beta risk. These occurrences cannotbe eliminated, but they can be reduced to the pointwhere the probability of their happening is small.One risk is usually more costly than the other, and thelimits are set accordingly. The limits are set far fromthe mean when the alpha risk must be avoided. Theyare set close to the mean when the beta risk must beavoided.It is standard practice in black-and-whiteprocessing to place the limit lines at three times thestandard deviation above and below the mean, or ±3s.The alpha risk is approximately 3 in 1,000 for limitsof ±3s. Before proceeding, it is necessary to definethe following two terms:Population—all possible results (happenings)in a certain processVariability—the amount of departure ofmeasurements (parts of the population) fromthe mean (average)Variability may be expressed in the followingways:2-26

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