With the advent of those particular light emitting diodes (LEDs) which generate bright-white light whereby such light emitting diodes (LEDs) can be used as viable and reliable illumination sources, many imaging systems, such as, for example, cameras, are replacing conventional incandescent illumination systems with LED-based illumination systems. The use of LEDs as a source of illumination for imaging systems has many operational advantages, as compared to conventional incandescent illumination systems, such as, for example, longer service life, lower power consumption, lower heat generation, and lower infrared color spectrum. On the other hand, white LEDs pose some operational challenges when viewed from an overall life-cycle perspective point of view. More particularly, for example, white LEDs are expensive as compared to monochromatic LEDs, such as, for example, red LEDs. In addition, relatively large quantities of the white LEDs are required in order to provide a requisite or sufficient amount of illumination. As a result, a white LED illumination system requires a relatively high acquisition and implementation cost relative to conventional incandescent illumination systems. Still yet further, white LEDs have an inherent operational characteristic of gradually losing their relative brightness levels during their service lives.
More specifically, white LEDs contain a phosphor substance that fluoresces so as to generate much of the white color spectrum, and overlying the phosphor substance is a clear plastic lens. It has been discovered, however, that over a period of time, the clear plastic lens tends to yellow due to the light frequencies that are generated, and in turn, the yellowing of the plastic lens effectively tends to lower the light output from the white LEDs. More particularly, there are several operational factors which not only lead to the aforenoted yellowing of the plastic lens, but in addition, such factors also affect the rate at which the plastic lens undergoes such a yellowing process. A first contributing factor comprises the amount of time that the LED is disposed in its ON state, a second contributing factor comprises the temperature of the LED, and a third contributing factor comprises the amount of current which is being conducted through the LED. For example, with reference being made to FIG. 1, there is disclosed a graphical plot which clearly illustrates the gradual deterioration or degradation of the RELATIVE LUMINOSITY, luminance, or brightness, of white LEDs, as a function of OPERATING TIME, under similar temperature conditions of 25° C., but under different current amperage conditions in milliamps. As can be readily appreciated from FIG. 1, when the LEDs are operated at a substantially higher current level, that is, for example, at 20 ma, as depicted by means of graph B, as opposed to 10 ma, as depicted by means of graph A, the onset of the deterioration or degradation of the relative luminosity occurs at an earlier point in operational time, with the ultimate result being that the luminosity of the LED decays to, for example, an unacceptable level within a shorter period of time so as to effectively define a substantially shorter service life for the white LED.
Continuing further, conventional imaging systems, such as, for example, cameras, normally contain at least one mechanism for operatively affecting the brightness of the illumination system, and therefore, in connection with the use of a white LED illumination system, such mechanism or mechanisms would effectively be capable of compensating for the aforenoted deterioration or degradation in the produced brightness of the illumination system. Such operative compensating mechanisms typically control exposure and comprise, for example, an iris control mechanism and a gain control mechanism. The iris control mechanism or f/stop adjusts and affects the aperture size so as to directly control the amount of light that is transmitted to and passed through the lens, while the gain control mechanism comprises an electronic adjustment that is applied to or impressed upon the video circuits of the digital camera that control the amplification of the video signals from their source, such as, for example, a charge-coupled device (CCD) sensor. When these two control mechanisms are properly set or adjusted, the exposure level of the imaging system is correct. It is to be appreciated, however, that both the iris and gain control mechanisms have practical limits which, in reality, affect or limit the extents to which the exposure levels can in fact be affected. For example, the iris control mechanism is limited by the size of the imaging system lens as well as the depth of field required by the system. The gain control mechanism is effectively limited by the amount of noise that is acceptable to, or which can be tolerated by, the system. As gain is increased so as to effectively compensate for low illumination levels, the noise is likewise increased. Accordingly, there is a point or limit beyond which gain can no longer be increased due to the fact that the corresponding noise levels would be too high and therefore unacceptable with respect to the desired imaging capabilities or characteristics of the system.
In light of the foregoing, it can readily be appreciated that all conventional imaging systems are therefore predeterminedly designed in such a manner that the iris and gain control settings have built-in margins or tolerances whereby the iris and gain control settings are not normally or originally operated at their upper or absolute limits so as to effectively provide for subsequent adjustments as will become necessary. A typical or conventional system will therefore initially operate at such “normal” levels until such time that the illumination, luminosity, or luminance levels characteristic of the system drop to such an extent that one or both of the iris and gain control settings must be adjusted so as to effectively compensate for such a drop or loss in the illumination, luminosity, or luminance level in order to in fact maintain proper system exposure parameters or levels. During the time that such adjustments are being implemented, the image quality, as measured or determined by means of the depth of field and noise characteristics, will be adversely affected, and eventually, effective exposure compensation terminates when the real or practical limits of the depth of field or noise are exceeded. The aforenoted procedures may be graphically appreciated from FIG. 2 which is a graphical plot of both RELATIVE LUMINOSITY and GAIN as a function of OPERATING TIME.
More particularly, it can be appreciated that the graphical plot of RELATIVE LUMINOSITY, or GENERATED LED ILLUMINATION, of FIG. 2 is substantially similar to the graphical plots illustrated within FIG. 1, that is, the LED illumination will in fact deteriorate or degrade as the service operating time of the LEDs increases. Correspondingly, for example, and separate and apart from any adjustments which may be made to the iris control mechanism, adjustments in the gain control mechanism may be accordingly implemented so as to effectively counteract, compensate for, or offset, such deterioration or degradation in the LED illumination levels. Therefore, in accordance with conventional imaging system operational techniques, when the LED illumination components are fresh or new, the gain control mechanism is intentionally set or adjusted to a predetermined operative level of, for example, approximately forty percent (40%) of the maximum obtainable gain, and as the LED illumination levels deteriorate or degrade over time, the gain levels are correspondingly increased so as to effectively counteract, offset, or compensate for such loss, deterioration, or degradation in the LED illumination levels. It is of course readily appreciated from the graphical plot of FIG. 2 that eventually, viable gain adjustments can no longer be implemented in view of the fact that the gain level reaches 100% MAXIMUM OBTAINABLE GAIN, meaning, that if the gain signals are increased still further beyond such level, the resulting noise levels effectively impressed upon the resultant imaging scans would render the same unacceptable or undesirable. Accordingly, it can be readily appreciated still further, as graphically illustrated within both FIGS. 1 and 2, that the LEDs will in fact continue to age relatively quickly. It is lastly noted, in conjunction with the graphical plot of the sensor gain adjustments, that such adjustments have been graphically illustrated in a stepwise manner, however, over a substantially extended period of time, such graphical plot will effectively exhibit a substantially linear increase in such sensor gain adjustments.
Continuing further, and in light of the foregoing, it can readily be understood that as a result of the relatively rapid aging of the LEDs, and in view of the fact that when the illumination levels of the LEDs therefore degrade or deteriorate to those levels which cannot effectively be corrected by means of the imaging system exposure controls, the illumination system must be replaced. Obviously, the economic impact of relatively high replacement costs, coupled with a foreshortened useful life expectancy effectively dictated by means of constantly deteriorating or degrading illumination levels, can have a substantial negative effect upon the implementation and operational costs of such a system over its entire service lifetime. Still yet further, it is likewise important, from a cost-effective point of view, to know, as accurately as possible, precisely when the LEDs will no longer be capable of delivering the requisite illumination levels such that the LEDs can be replaced at the appropriate time, as opposed to being replaced prematurely and therefore needlessly, or alternatively, as opposed to being replaced after such appropriate time has occurred whereby the system would have to be operated under less than desirable or acceptable illumination levels. In addition, in order to prevent the need to operate the system beyond the appropriate replacement time, such as, for example, when replacement LEDs may not be readily available, a needless oversupply or large inventory of LEDs would otherwise need to be provided.
A need therefore exists in the art for a new and improved technique by means of which the substantially rapid aging of LED illumination sources, which results in a substantially rapid decay, deterioration, or degradation in the illumination levels of the LED illumination sources, can effectively be forestalled or delayed such that the real or effective service life of the LED illumination sources may be enhanced so as to, in turn, significantly reduce system implementation and operating costs, and wherein further, a need exists in the art for a new and improved technique by means of which the true service life of the LED illumination sources may be more accurately determined, forecasted, and predicted such that operator or maintenance personnel can more accurately monitor the illumination levels of the LED illumination sources and effectuate the replacement of the LED illumination sources as necessary at the appropriate times.