Polymer Optics Ltd. 6 Kiln Ride, Wokingham Berks, RG40 3JL, England Tel/Fax:+44 (0)1189 893341 www.polymer-optics.co.uk Future Electronics EZ-Color Seminar Autumn 2007 Colour Technology Mike Hanney Technical Director Polymer Optics Limited Polymer Optics Limited is a Certified Future Electronics LUXEON Solution Partner
Overview of Presentation Physiology of the Human Eye and Colour Detection Chromaticity CIE colour space Black Body Radiation Colour Temperature Correlated Colour Temperature (CCT) Colour Rendering Index (CRI) Colour Mixing POL Colour Mixer Technology
The Human Eye The retina is the light sensitive surface at the back of the eye which contains millions of light sensitive nerve endings. These nerve endings are present in two types - RODS and CONES. The RODS are sensitive to the amount of light, so they respond to the relative brightness of the incident light. The CONES respond to colour and are present in three types with sensitivities to Red, Green and Blue light.
The Human Eye The CONES are highly concentrated in the Fovea (the central area of focus of the eye). There are very few colour photo-receptor CONES outside the central fovea. Whereas, the RODS are concentrated in the peripheral area. Therefore, human peripheral vision is primarily in black and white. This can be realised since at night you need to look to the side of a dim object to see it more clearly.
The Human Eye The theory of three colour vision was postulated by Helmholtz in the mid 19th Century. Hermann von Helmholtz (and before him Thomas Young) proposed that all visible colours are seen as a result of stimulating differing numbers of responses from red, green and blue cones in the retina. This is the core of what is now called the Young-Helmholtz Three Component Theory Short wavelength (S cones) contain cyanolabe mostly sensitive to "blue violet" wavelengths at around 445 nm. Medium wavelength (M cones) contain chlorolabe mostly sensitive to "green" wavelengths at around 540 nm. Long wavelength (L cones) contain erythrolabe - mostly sensitive to "greenish yellow" wavelengths at around 565 nm.
The Human Eye However, human colour perception is not that simple since variations can occur due to: Amplification changes in cone receptor signals. Wavelength changes in cone receptor sensitivities. Colour deficiencies due to absent cones or different cone distributions (colour blindness) Retinal photographs with coloured cone receptors Roorda et al, Nature, 397, 1999
The Human Eye Prevalence of Colour Blindness Monochromacy Rod monochromacy (no cones) Male - 0.00001% Female - 0.00001% Dichromacy Protanopia (L-cone absent) Deuteranopia (M-cone absent) 2.4% 1% to 1.3% 1% to 1.2% 0.03% 0.02% 0.01% Tritanopia (S-cone absent) 0.001% 0.03% Anomalous Trichromacy Protanomaly (L-cone defect) Deuteranomaly (M-cone defect) Tritanomaly (S-cone defect) 6.3% 1.3% 5.0% 0.0001% 0.37% 0.02% 0.35% 0.0001%
The Human Eye "Ishihara Colour Vision Test", developed by Dr. Shinobu Ishihara. People with normal colour vision should see an 8 Red-green colour vision deficiencies should see a 3 Total colour blindness should not see any numeral. People with normal colour vision should see a 5 Red-green colour vision deficiencies should see a 2 People with normal colour vision and those with all colour vision deficiencies should read the number 12.
Chromaticity In the 1920s, Wright and Guild independently conducted a series of experiments on human sight which laid the foundation for the specification of the CIE XYZ colour space. The experiments were conducted by projecting a test colour and adjacent to it was projected a colour that the observer could adjust. The adjustable colour was a mixture of three primary colours, each with fixed chromaticity, but with adjustable brightness. Not all test colours could be matched using this technique, so when this was the case, a variable amount of one of the primaries could be added to the test colour. In these cases, the amount of the primary added to the test colour was considered to be a negative value. The CIE 1931 RGB Colour Matching Functions
Chromaticity The from the RGB Colour Matching Function three relative colour strengths can be plotted in a three-dimensional space, which defines the properties of Hue, Chroma and Lightness. These properties are plotted in the CIE XYZ Colour Space (CIE 1931 Colour Space) Refined into the CIE L*a*b 1976 Hue Hue What is the colour? Chroma How saturated is the colour? Lightness how bright is the colour?
Chromaticity However, the concept of colour can be divided into two parts - lightness and chromaticity (hue and chroma) The CIE XYZ colour space was deliberately designed so that the Y parameter was a measure of the lightness (brightness or luminance) of a colour, such that the chromaticity of a colour could then be specified by the two derived parameters x and y which are functions of all three X, Y, and Z tristimulus values: The derived colour space specified by x, y and Y is known as the CIE 1931 xy Colour Space. Note: The chromaticity diagram is used to specify how the human eye will experience light of a given spectrum. It doesn t specify colours of objects (or printing inks, etc.), since the chromaticity observed while looking at an object depends on the illuminating light source as well.
Chromaticity C.I.E 1976 Chromaticity Diagram Since the CIE 1931 diagram was developed, two revisions have been made in 1964 and 1976. They plot the same information scaled differently. The advantage of the CIE 1976 diagram is that the distance between points is approximately proportional to the perceived difference in colour.
Black Body Radiation The outer curved boundary is the Spectral Locus, with wavelengths shown in nanometers. The Black Body Radiation colour temperatures can be plotted on the CIE 1931 xy Chromaticity Diagram and the line plotted through them is called the Black Body Curve or Planckian Locus Colours on this locus between 2,500K and 10,000K are considered to be white.
Black Body Radiation When an object is heated to a high temperature it will glow. The colour of the glow with change through dull red - brighter red - bright orange - brilliant white. Although the brightness will vary from one material to another, the spectral distribution is essentially universal for all materials and is dependant on the temperature only. This is known as Black Body' Radiation, and is described by Planck's Radiation Law: Spectral energy density = U(λ,T) = 8πhcλ-5 / (ehc/λkt-1) Where λ is the wavelength (m) T is the temperature in Kelvin (OK) h = 6.626 10-34 J s (Planck's constant) k = 1.381 10-23 J K-1 (Boltzmann's constant) c = 3.0 108 m s-1 (speed of light)
Black Body Radiation Blackbody Spectral Emittance Curves The plotted temperatures of 2860 K, 5000 K and 6500 K correspond to the CIE illuminants: A D50 D65 As the blackbody temperature increases, the peak emittance increases and shifts from infrared to ultraviolet wavelengths.
Correlated Colour Temperature (CCT) Since most artificial light sources do not have a smooth spectral curve which exactly matches a black body emission curve, the light source is an approximation to the closest black body colour temperature curve. In this case, the artificial light source provides the closest correlation to the visual appearance to the black body emission curve, so is called the Correlated Colour Temperature (CCT) of the light source. The lines crossing the Planckian locus are lines of constant correlated colour temperature (CCT).
Colour Rendering Index (CRI) Colour Rendering Index is a method for describing the effect of a light source on the colour appearance of objects, compared to a reference light source of the same Correlated Colour Temperature (CCT). Provides a means of gauging the quality between light sources emitting light of the same or very similar CCT. In 1965, the CIE introduced a standardised measuring method to be able to objectively compare the colour rendering properties of light sources. This method calculates the colour change of 14 test colours under the light source being tested relative to the colours measured under a reference illuminant. The General Colour Rendering Index (Ra) is a measure of the average appearance of the first eight indices colours. The remaining 6 colours (numbered 9 to 14) are used to provide additional information about the colour rendering properties of the light sources and is called the Special CRI (Ri).
Colour Rendering Index (CRI) R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 Light Greyish Red Dark Greyish Yellow Strong Yellow Green Moderate Yellowish Green Light Bluish Green Light Blue Light Violet Light Reddish Purple Strong Red Strong Yellow Strong Green Strong Blue Interestingly, in Japan, the standard can consist of 15 colour indices, where the 15 th sample is a Japanese skin complexion tone! R13 R14 R15 Light Yellowish Pink (Human Complexion) Moderate Olive Green (Leaf Green) Japanese Complexion
White Light LED Illumination Options Too little Dark Red? 3200K Warm White LED Too much Blue? Good CRI of around Ra 90 CRI is reduced by: Too much Blue and too little Dark Red
White Light LED Illumination Options R G B Poor colour rendering ~65 CRI Ra
White Light LED Illumination Options Too little Cyan Too little Dark Red 5500K Cool White LED Low CRI of around Ra 70 CRI reduced by: Too little Cyan and too little Dark Red Spectral distribution can be more closely matched with addition of Cyan and Red LEDs
White Light LED Illumination Options C R G B W O Very good colour rendering possible ~96 CRI Ri
Colour Rendering Index (CRI) For a near monochromatic light source, such as a sodium vapour lamp, the CRI will be almost zero. Typical cool white fluorescent lamps have a CRI of 62 Ra R+G+B mixing will give approximately a Ra 65 CRI, due to narrow spectrum of each LED 5500K Cool white LED typically has a Ra CRI of around 70 Newer "tri-phosphor" fluorescent lamps can have CRIs of around 80 to 90 Ra 3200K Warm white LED around Ra 90 CRI wider phosphor spectrum improves CRI Mixed R+G+B+A+W claimed to give a CRI Ri value of ~96, with values of between 85 and 98 across each of the 14 colour indices. Incandescent light sources, since they are very close to black body radiators, have high CRIs of nearly 100.
POL Standard LED Optics Range 120-6 O 124-25 O 126-6 O x25 O 141 - Concentrator 129 - LUXEON V 6 O Collimator 152-6 O 7 Cell Collimator 128 - Holder LUXEON I and III Stars 121 - Holder LUXEON I, III and V Emitters and LUXEON I Stars 151 - Holder LUXEON K2 Emitters 153-25 O 7 Cell Collimator 154-6 O x 25 O 7 Cell Collimator 122-6 O 7 Cell Collimator 125-25 O 7 Cell Collimator 140-7 Cell Concentrator
POL Standard LED Optics Range
Extended Light Source Considerations f I f O In a simple imaging lens system, the object is magnified in the ratio of the two focal distances f O and f I which is equal to the ratio of the object and image size. Object size = f O Image size f I In the TIR lens, or any simple lens, the same effect on the beam divergence is produced by the effective focal length of the lens and the size of the LED emission area.
Extended Light Source Considerations Luxeon I and III die size ~1 mm sq. Luxeon K2 die size ~1 mm sq. Luxeon Rebel die size ~1 mm sq. Luxeon V die size ~2.2 mm sq. (4 dies in single package) How not to do it? Some LED manufacturers place many dies in a single package to try to provide very high light flux devices. However, their large extended source sizes of many millimetres require very large optics to provide adequate light collection and beam control. In many applications, the system becomes much larger than using an array of Luxeon LEDs fitted with smaller individual optics.
RGB Colour Mixing A number of multi-die RGB LEDs have emerged onto the market The separation of the dies produces angular separation of each colour Although the beams will diverge and overlap with distance, considerable colour separation will be seen around the edges of the core beam R G B
RGB Colour Mixing The alternative is to widen the beams to achieve better mixing Colour mixed spot light can t be achieved Not such a useful beam R G B Narrow angle RGB mixing difficult!
RGB Colour Mixing Compact POL Colour Mixer Optic LUXEON I, III, K2 and Rebel devices Collimated light path from each LED is split into three parallel paths maintaining narrow angle beam collimation. Light output from 3 LED s (RGB) appears like 9 LEDs interleaved In this way, good colour mixing is achieved over short ranges and with a narrow collimation angle
RGB Colour Mixing 1.2 1 0.8 0.6 0.4 red blue green white (RGB) Red Green Blue 0.2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Results measured with 3 x K2 Luxeon LEDs Red, Green, Blue Red/Blue Green/Blue Red/Green White Red/Green/Blue
RGB Colour Mixing 25 Deg Diffuser - 161 6 x 25 Deg Line Diffuser - 162 Soft Beam Diffuser - 163 Can be used with Luminit LSD filters and colour filters Plain Filter Holder - 160
RGB Colour Mixing RGB Shower Optic for Hansa, GmbH
Colour Mixing and Control White Point Control White Balance One of the most demanding operations of the RGB system will be to maintain an acceptable mixed white colour The three proportions of the primary light sources can be controlled to produce a white coloured light called the White Point which for a pure white will be located at x=0.33, y=0.33 However, the peak wavelength and the output flux of each LED will change with temperature and binning. So as the illumination system changes temperature with different drive conditions or environment, the White Point will move to a different colour Therefore, to maintain a stable colour control, the electronic drive needs to be calibrated and able to compensate for temperature drifts to intelligently hold the set Colour Point
Colour Mixing and Control Drives for LEDs Binning Thermal feedback Photometric feedback
Polymer Optics Limited is a Certified Future Electronics LUXEON Solution Partner Polymer Optics Limited 6 Kiln Ride, Wokingham, Berks., RG40 3JL, England Tel/Fax:+44 (0) 1189 893341 www.polymer-optics.co.uk Thank you