Much of these new 'findings' and demand for a revamped metric for rating visually pleasing CRI/Colro Temp was pretty much confirmed by research done by the 'quack' Aten Imago, a former CPF member.
Here's the original article Presented to the IEEE, LRI :twothumbs Fully digested by HarryN
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Theory & Method of LED Color Blending For Enhanced Nocturnal
(Scotopic & Mesotopic) Subject Rendering
Authors:
Äten Imägo
Imägo Metrics - Bethesda, MD - USA
M.P.El-Darwish
Geneva, Switzerland
Version 2.0
October 24, 2005
"Optimizing Human / Technology Interaction Performance"
Abstract
This document lays out the theoretical and research basis for the optimization of LED color blending for the purpose of better aligning the requirements of night adapted human vision (AKA Scotopic/Mesotopic Vision) with the artificial LED source illuminations prevalent in personal illumination devices such as flashlights, bicycle lights and personal reading lamps.
Keywords: Scotopic,Mesotopic,Photopic, Human Factors, Usability, Performance Optimization, Dichromasty, Night Adaptedness, Luxeon, LED, Flashlight, Reading Lamp, Vision, Amber, Straw, White
CONTENTS
EXECUTIVE SUMMARY 3
A BRIEF HISTORICAL OVERVIEW OF THE CASE 4
THEORETICAL BASIS 5
Illumination brightness and contrast as influencers 6
Conditions for optimized supplemental illumination 6
Shortcomings of current LED emitters 8
Methods for creating Amber/Straw/White light from combined beams 8
FUTURE OPTIONS 9
Design community barriers to blended LED theory 9
Current design solutions in development 10
REFERENCES 12
Executive summary
This document sets forth the theory and method associated with the blending the light of two or more high flux LEDs into a combined beam color that has a Color Rendering Index (CRI) as well as wavelength that is harmonized to the requirements of persons performing tasks in low or no ambient light conditions. The need for such a method is due to the lack of a suitable single emitter LED from the available manufactured stock.
Who will benefit from combined color LED arrays?
o Individuals that have benefited from the wavelength of light produced by incandescent lights in the 3,000ºK to 4,000ºK range.
o Individuals seeking variable combination beam colors
o Stakeholder communities that are developing large scale environmental lighting - using LEDs to replace incandescent lights.
o Manufacturers looking for ways to expand the markets for their products.
Benefits
What benefits can be expected?
o Abandonment of incandescent bulbs
o Reduced cost of bulb replacement
o Elimination of color interaction issues resulting from the use of current LEDs
o Introduction of nuanced white combination arrays
o Increased sales opportunities
Productivity Enhancements
What benefit in productivity will result from this process?
o A competitive edge: Manufacturers, Distributors and Customers will flock to this solution, until a single optimum emitter bin color is introduced
o Enables end-users to experience the same advantages from an combined LED array that is currently available to users of incandescent bulbs
o This is a gap filling solution that will bridge a potentially long cycle in the High Flux design process
Improvements Realized in Actual Use
Improvements will be realized as a result of:
o Reduced eye-strain
o Reduced user errors that can slow productivity down
o Increased speed of object identification as well as spatial navigation
The argument for an Amber/Straw/White VS Daylight LED source
A brief historical overview of the case
Taken from a layman's point of view and then underscored by empirical science, the argument for an Amber/Straw/White light for night-adapted illumination, versus a Daylight white is thus:
The current human visual sensory array of Retinal Rods & Cones evolved from a biologically adapted array that deliberately took advantage of the wavelength of light produced by flames. Flames such as those from camp fires, torches and later- oil lamps, candles and gas burning street lamps- invariably produced light that was Amber/Straw/White in wavelength.
This represents an adaptive evolution of the retinal sensory array to coincide with the available wavelength of light. The result is that our visual system became calibrated to optimize night vision to this wavelength-that in layman's terms we have called Amber/Straw/White.
The scientific deconstruction of this evolutionary trend leads to what we currently know about scotopic and mesotopic vision. What is known about scotopic vision is that Retinal cells called Rods are sensitive to very low levels of illumination and are responsible for our ability to see in dim light (scotopic vision). Rods contain a pigment with a maximum sensitivity at about 510 nm, in the green part of the spectrum. The rod pigment is often called visual purple since when it's extracted by chemists in sufficient quantities the pigment has a purple appearance. Scotopic vision is completely lacking in color; a single spectral sensitivity function is color-blind and thus scotopic vision is monochromatic.
The situation becomes more complex when nocturnal vision combines night adapted or scotopic vision, with full color or photopic vision. Mesotopic vision is the scientific term for a combination between photopic vision and scotopic vision in low but not quite dark lighting situations.
For example: the combination of the higher total sensitivity of the rods in the eye for the blue range with the color perception through the cones results in a very strong appearance of blueish colors (eg. flowers) around dawn.
The human eye uses pure scotopic vision in the range below 0.034 cd/m2,and pure photopic vision in the range above 3.4 cd/m2.
In the case of a general subject field that is a field of low contrast darkness that is then pierced by a smaller and localized region of illumination, the visual system works to resolve the brightness and color contrasts by combining the use of Rods and Cones. The resulting compromise illumination color that supports this effort is Amber/Straw/White - light near the 580nm region of the spectrum. This is the most productive supplementary illumination color space for both scotopic and mesotopic vision.
Caveat Emptor - Just as the anthropological record depicts divergent geographical trends in the evolution of human sensory perception, we do not contend that the generalized statements made about human scotopic, mesotopic or photopic vision are absolute or universal. The nomadic peoples of the arctic circle, for example, seem to have evolved an ability to perceive color nocturnal contrast differently from those living on at equatorial latitudes.
Theoretical basis
The foregoing historical and scientific statements pertain to circumstances in which there is no ambient environmental light and the sole source of illumination is a highly localized one, such as might be generated by a flashlight, torch, candle or spotlight. The situation becomes more complex when one considers the following examples:
o Low level and general blueish illumination such as may be produced by the full moon on a clear night
o Low level and general Amber illumination such as might be produced by certain sodium vapor street light arrays
In such cases, the visual system resolves these 'casts' so that they are not perceived as casts at all. When a bright and localized beam is then projected onto a target subject that is surrounded by a diffuse base illumination color, the visual system is shocked and the newly introduced light appears to have a cast that is typically the spectral compliment of the prevailing light. As an example- the introduction of an otherwise neutral and spectrally pure white beam of light from a flashlight into a scene that is bathed is the amber hue of sodium vapor street lights (AKA 'Crime Lights') creates a discontinuity in signals sent by Rods & Cones to the brain- resulting in a perception that the Daylight beam is actually Blue in hue. This discontinuity is greater when the localized beam is smaller. Expressed differently- as the field of supplemental illumination widens and exposes more retinal cells (Cones) to its wavelength, so in turn will the resolution or 'leveling' be more in favor of this localized beam color. In practical terms this means that in a situation of a low level of general illumination plus a localized supplemental light- the supplemental light wavelength should match that of the general illumination. Interestingly, this statement holds true in broad daylight as well!
In cases where the general illumination is so low as to be negligible, then the sole and localized illumination source is optimally in the 580nm region of the spectrum.
Illumination brightness and contrast as influencers
Thus far we've only talked about the interactions of illumination source wavelengths in affecting perceived subject color and ease of night-adapted vision. Another part of the equation relates to the relative contrast and brightness of the scene and the target or 'focus' region within that scene.
I've already touched on the discontinuity that occurs when a bright light of a different wavelength is projected into the scene. I'd like to comment on the added effect of brightness on the visual system's interpretation of colors.
The prevalent thinking has been that the desirable torch or flashlight illumination is one that is closest to neutral daylight in color (approximately 5,400ºK) and brightness is as high as technically possible. Some manufacturers have gone so far as to insist that a bright narrow spotlight that is 5,400ºK in hue is the best solution to most personal illumination challenges. I beg to differ.
Introducing a highly localized (spot) beam of light into a scene bathed in darkness creates havoc for fully night-adapted vision. Choosing to make that beam Daylight balance by default, compounds the shock and reduces night adaptedness which results in increased perceived contrast between the spotlit target and the surrounding scene. This predicament not only makes effective object identification more difficult and less effective- it also 'bleaches' Rods and renders them null- making perception of details in the surrounding scene very difficult if not impossible. In summary, we can make the following statements with regard to localized light brightness, focus and color.
Conditions for optimized supplemental illumination
o In cases where effective perception of shape and form in the prevailing and surrounding darkness is important:
o Supplemental illumination level or brightness should only be just as bright as is necessary in order for the user to perceive sufficient detail in the subject for effective identification
o The transition from focus (spot) to corona, penumbra and then surrounding darkness- should be as gradual as possible
o If there is negligible or no ambient illumination, then a localized torch or flashlight beam color of Amber/Straw/White (580nm) is ideal
o If there is a predominant ambient color, then matching that color will mitigate against a chromatic discontinuity that will likely result in the perception of a cast
o The axis of illumination should be slightly off dead-center from the user's direct line of sight
o In cases where the ambient illumination is broad daylight and the subject is located in a pocket of darkness:
o The supplemental illumination level can be as bright as or slightly brighter than the ambient brightness level
o The transition from focus (spot) to corona, penumbra and then surrounding brightness- may be abrupt
o The supplemental illumination may be daylight or as close a match in color to that of the surrounding ambient scene
o In such cases, there is no advantage in using light that is the same as that used for night and indeed doing so would be counterproductive
o The axis of illumination should be slightly off dead-center from the users direct line of sight
o In cases where the subject area is a significant portion of the overall scene
o The supplemental illumination should cover as much of the target area as possible
o The light should be as diffuse as possible
o The axis of illumination should be slightly off dead-center from the users direct line of sight
o In cases where the entire scene is an enclosure such a small room, passageway or container with reflective walls and a high degree of subject color differentiation is important
o The supplemental illumination is more effective if it's a neutral white or slightly warm white in the 5,200ºK to 5,400ºK range with a Color Rendering Index (CRI) of between 90 and 100
o The transition from focus (spot) to corona, penumbra and then surrounding darkness- should be as gradual as possible
o The light should be as diffuse as possible
o The axis of illumination should be slightly off dead-center from the users direct line of sight
o The supplemental illumination level can be as bright as or slightly brighter than the ambient brightness level
Shortcomings of current LED emitters
Currently, LEDs are available in a wide range of colors. A narrow range of white LEDs have filled market needs for illumination that supplements or supplants the most neutral and highest color rendering Daylight. This is a boon to many industries that depend on repeatable, consistent and long-lived Daylight balanced artificial light.
Unfortunately, the need for an Amber/Straw/White LED has been neglected for lack of a scientific basis for such a need. In addition, research and development efforts have understandably been focused on serving the growing demand for large-scale end uses, rather than the smaller demands of the personal illumination markets.
The nearest to an ideal flashlight LED color for applications such as the outdoors markets, law enforcement, security and general consumer using such lights outdoors (as differentiated from high color differentiation applications), has existed in the form of an Amber Lumileds™ Luxeon™ emitter. A warmish white X bin LuxeonV emitter has also been a near proxy color. The Amber Lumileds™ Luxeon™ emitter is far too 'Orange' in cast to be useful for general applications. As a result, it is only through mixing of light from an Amber Lumileds™ Luxeon™ emitter with light from other White Lumileds™ Luxeon™ emitters that an output brightness and color optimized blended color can be achieved. This is the core of our theory.
Methods for creating Amber/Straw/White light from combined beams
There are two approaches for creating the target Amber/Straw/White beam at 580nm from Luxeon LEDs
Approach 1 - Co-located flashlights
The most obvious approach, and one that I used in initial experiments, simply entailed binding together three or four identical flashlights- one using a Lumileds Amber Luxeon LED and the remainder containing White Luxeon LEDs of the same color or BIN and the same output. The axes of the resulting cluster of lights were adjusted so that all the beams would overlap- creating the single blended beam.
Approach 2 - Co-located emitters
A more efficient and effective approach is to cluster either two, three or four white emitters around a single amber one and to then insert this array into a large flashlight. This latter method is currently popular in the design of products that seek to multiply the generated output from a fixed physical space inside a flashlight's head. Most typically, each emitter is contained in it's own reflector.
The benefits of using Approach 2 include:
o Ability to nuance the relative output between the Amber and White emitters
o Ability to create massive banks of light for large-scale applications
o Ability to replace one or more emitters to create special effects
o Smoother beam mixture
o Resultant beam is more akin to that from an incandescent bulb flashlight
Future Options
In the future, Lumileds may introduce an LED or series of LEDs that emit beam colors that are comparable to those currently produced by incandescent bulbs. In the interim, another evolutionary stage may involve the use of the new K2 series of Luxeon III emitters.
This K2 series of emitters efficiently dissipates heat, a shortcoming of the prior generation of Luxeon III.
One advantage of this to the flashlight design engineer may include the ability to cluster a group of three, four and five emitters is very near proximity and in such a fashion that they may share one reflector. Though not necessarily envisioned for traditional flashlight designs, such arrays could be a boon to designers of illumination for other fixtures such as reading lamps, bicycle lights, camcorder lights, area lights and so forth. This would have the following advantages
o Individual emitter/reflector pairs would not require as precise an alignment to remaining emitter/reflector pairs
o A variable focus relationship between the cluster and single reflector could be established.
Design community barriers to blended LED theory
The design community in any domain of industry is responsible for driving changes that ultimately benefit the user/consumer. Understandably, barriers to change exist within the flashlight community. The following is a list of possible design community barriers and how they may be overcome
o Cultural: The power of mass communication in disseminating information can result in 'mob mentality' or 'group-think', which can in turn result in sub optimal designs being adopted by masses of manufacturers. Once the concept of the LED as a light source for flashlights took hold, it was only a matter of time before cultural norms emerged that would set the stage for 'standards' of excellence in all aspects of LED use in flashlights. Changing the tone of these standards is critical to winning community adoption of a better design solution
o Technical: Design is often constrained by technical limitations. Before the advent of user-centered design that took human factors into considerations, nearly all efforts in the design of LED based flashlights have been concentrated on increasing brightness, achieving as neutral a white beam as possible and optimizing battery usage. By changing the attitude to beam colors to include Amber/Straw/White as an option; previous constraints cease to be an issue- paving the way for wider adoption of the blended beam solution
o Strategic: Manufacturers, while slow to making changes on a large scale- do notice design trends in the user community. Up-scaling adoption at the user level should drive changes up the value chain to the manufacturers- resulting in exponential changes across the entire flashlight landscape.
Current design solutions in development
Based solely on the 'prior art' trial experiments of imago Metrics's test of the multiple-lights with one being a Lumileds Luxeon III Amber, Andrew Wynn Rouse of Wynn Bright flashlights -with a mult-emitter light already in production immediately saw the possibility of replacing one of the four white emitters with an amber emitter which was furnished courtesy of Future Electronics - distribution partners for Lumileds, manufacturers of the Luxeon brand of LED emitters.
The website containing the original pictures is here:
http://rouse.com/RT4 (BAM+)
There are BAM+ beam shots here:
http://rouse.com/beams
In the current design prototype, we did not have the physical space for a symmetrical array as is specified for the working prototype. Even so, the slight misalignment of the amber emitter in the array turned out not to be conspicuous in real-world use- though for aesthetic reasons we would like to continue development of a symmetrical array such as one amber surrounded by three white emitters (three points of an equilateral triangle) with a fourth, the amber emitter at the center.
One symmetrical design currently in alpha stage, is a flashlight using one amber emitter flanked by two white ones. This design is named the MiniMighty flashlight.
The results of preliminary trials show that a high-power LED array can match the illumination quality of a medium/high power 'hotwire' incandescent lights, with all the advantages of using LEDs and none of the disadvantages associated with incandescent lights.
Prototypes in progress shall serve as proof of concept and as envisioned, will fill a gap currently unfilled by any solution other than filtration.
In addition, future trials will include the ability to variably nuance the relative output of amber versus white emitters, so as to affect a range of color temperatures from 4,000ºK to 3,200ºK in addition to permitting variable brightness levels. Such fixtures could have far ranging applications in interior architectural illuminations, where nuanced illumination hues can have a bearing on the intended mood of the interior design setting as well as in supporting effective task illumination.
References
Ware, C (2000) Information Visualization- Perception for Design 103-149.
Salvendy,G (1997) Handbook of Human Factors and Ergonomics (672,875-876,1737,1699,62-64,669,870,1735,862-863,176,872,59)
Morris,A (1950)Visual acuity at Scotopic Levels of Illumination
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© 2005 Imago Metrics LLC - All Rights Reserved
Finally folks are coming around to understanding that interactions of localized ( ie flashlight, display light, area light) illuminations with ambient illumination, context of use and level of light are the key drivers for determining ideal CRI and not blind engineering combined with manufacturing capacity.
