Electric lamp and discharge devices: systems – Plural load device systems
Reexamination Certificate
2002-03-28
2004-07-06
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural load device systems
C362S545000, C362S800000, C358S475000, C358S509000
Reexamination Certificate
active
06759814
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of illuminators suitable for accurate calorimetric work in electronic imaging and traditional photographic design and production environments. More particularly, the invention concerns illuminators using solid-state emitters and having independent control of both the output spectral characteristic and power level.
BACKGROUND OF THE INVENTION
Illumination sources are used in the digital still camera (DSC) development and production environments to support a number of different tests and/or calibrations. For example, it is usually necessary to set the relative gains of the DSC's various color channels to achieve equal channel responses for a given illuminant color temperature. This procedure is often referred to as “white-balancing” the camera. It is important to use an illumination source during this adjustment procedure that accurately mimics the spectral characteristics of the illumination condition under which the camera will actually be used. Examples of white-balance settings that may be selectable on a DSC are daylight, fluorescent, tungsten, and flash. The term ‘illuminator’ and ‘illumination source’ will be used interchangeably throughout this document and should be understood to mean the same thing.
In addition to accurately reproducing a particular illumination spectral power distribution (SPD) characteristic, an illuminator may also be called upon to provide an adjustable output level which does not, in turn, impact the spectral shape of the output. Some examples of the need for this capability are the testing or calibration of a DSC exposure control system or the mapping of the linearity of an imager or DSC tone-scale. This capability is also useful for determining the density versus log-exposure characteristic of photographic films. Usually, the only way to adjust the output level of traditional illumination sources, without changing their spectral shape, is to employ either an adjustable mechanical aperture or a series of selectable neutral-density filters.
Illuminators are used in conjunction with either reflective or transmissive types of color charts to perform calorimetric calibration of a DSC. This procedure requires not only an illuminator with a high-quality spectral output but one whose output spectral characteristics are stable over time. In addition, the spectral characteristics of the color chart must also be stable over time Unfortunately, the aging characteristic of most lamps requires frequent illuminator recalibration in order to maintain accurate results. Physical color charts are often expensive to fabricate and suffer from fading effects that require periodic re-measurement. In addition, it is sometimes difficult to provide uniform illumination over an entire chart and this non-uniformity will impact the DSC calibration. It will be shown later that the present invention allows the color chart to be eliminated entirely, thereby reducing costs and improving calibration accuracy.
One popular illuminator technology is the quartz-tungsten-halogen (QTH) lamp. QTH illuminators typically operate at a color temperature between 2800 and 3200 Kelvins although higher color temperatures can be achieved by a QTH illuminator using appropriate color conversion filters between the illuminator and the DSC. The QTH illuminator is popular because of its relatively low cost and it's smooth spectral characteristic. Unfortunately, the QTH lamp technology also has many disadvantages. QTH lamps age quickly and must typically be replaced at intervals ranging from 40 to 200 hours depending on how the lamp is operated. Calibration of the lamp must be checked frequently, usually once or twice a day in a production environment, to ensure consistent product quality. QTH lamps also output a significant amount of energy in the infra-red (IR) region and this requires the use of expensive, heat resistant glass filters to block this unwanted energy from the illuminator output.
Halide-metal-iodide (HMI) and xenon lamps are a popular choice for simulating daylight illumination because of their higher correlated-color temperatures. These lamp types also suffer the disadvantages of aging and high output in the form of IR and heat. Both lamp types exhibit some amount of discrete line spectra that may be objectionable for some applications. Xenon lamps also suffer from “arc wander” which can affect spatial and temporal uniformity of the illuminator output.
Fluorescent lamps are available with correlated-color temperature ratings suitable for simulation of indoor incandescent as well as outdoor lighting conditions and exhibit a useful life of several thousand hours. One of the disadvantages of fluorescent lamps is the discrete line structure introduced into the SPD characteristic by the Mercury vapor used to excite the phosphors. The mercury line structure is modulated by the AC waveform of the lamp drive current and this can cause a potentially undesirable variation in temporal output of the lamp.
All of the illumination technologies described thus far require a warm up period before their output characteristics stabilize and, typically, this warm up period is on the order of 30 minutes. Even fluorescent lamps, which ignite quickly, require that the glass envelope reach a certain temperature for optimum operation. Because of this warm up characteristic, these types of illuminators must be left in the on-state and a shutter mechanism must be used in conjunction with the illuminator to perform DSC or imager measurements in the dark. The shutter mechanism can be quite expensive when it is also required to accurately control the exposure time on the device or film under test.
In recent years, light-emitting diode (LED) technology has advanced to the point where these devices are now a viable alternative to the traditional illumination technologies used in many applications. LEDs exhibit a turn-on time on the order of 100 nanoseconds, obviating the need for a separate shutter mechanism. Furthermore, because of their stable spectral characteristics, LEDs can be used in conjunction with pulse-width modulation (PWM) to accurately vary output level, thereby eliminating the need for neutral density filters. Appropriate application of PWM to a set of LEDs having a plurality of different peak wavelengths allows the synthesis of different SPDs, thereby eliminating the need for conversion filters.
U.S. Pat. No. 6,127,783 issued Oct. 3, 2000 to Pashley et al., and entitled, “LED Luminaire With Electronically Adjusted Color Balance,” describes a luminaire constructed from red, green, and blue LEDs to produce light of different colors based on the mixing together of suitable amounts of the three LED types. The Pashley method relies on the tri-chromatic color theory wherein three color “primaries” with suitable chromaticities in the Commission Internationale De L'Eclairage (CIE) x-y chromaticity diagram are used to create any color whose chromaticity falls within the triangular gamut defined by the chromaticities of the three primaries. A good discussion of tri-chromatic theory and use of the CIE chromaticity diagram can be found in the book
Measuring Colour
by R. W. G. Hunt (ISBN 0-470-20986-0) in sections 2.4 and 3.3, respectively. Pashley controls the current through the individual LED color channels to adjust the luminous output of each color channel and this has the desired effect of changing the chromaticity of the overall color mixture. The Pashley method suffers from a number of drawbacks when considered in the context of DSC or imager testing and calibration. First, the use of three narrow-band primaries, such as those produced by LEDs, may be inadequate for critical DSC calorimetric calibration because the DSC cannot discern colors in the same way as the human visual system does. This problem arises due to the fact that the spectral sensitivities of the DSC are not color-matching functions (CMFs), i.e. they are not directly related to the spectral sensitivities of the human visual system. During DSC calorimetri
Enge James M.
Pillman Bruce H.
Vogel Richard M.
Wanek Erin S.
Bailey, Sr. Clyde E.
Eastman Kodak Company
Philogene Haissa
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