Visibility, Performance and Perception. Cooper Lighting

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1 Visibility, Performance and Perception Kenneth Siderius BSc, MIES, LC, LG Cooper Lighting 1

2 Vision It has been found that the ability to recognize detail varies with respect to four physical factors: 1.Contrast between the details of a task and its immediate background 2.Luminance of the task 3.Size of the task 4.Time of viewing 2 Each factor is sufficiently dependent upon the others. A deficiency in one, within limits, may be compensated by augmenting one of more of the others. Only contrast, size, and, in certain instances, time, are inherent in the task itself.

3 Understanding the Problem The eye detects an object by the contrast it creates between itself and the surrounding background for a particular property: Size, time, contrast or luminance. Visibility is at a maximum when the luminance contrast (and color contrast if present) of details with the background is greatest. There is a minimum luminance contrast at which the eye detects an object. This minimum threshold is dependent on the Luminance contrast as well as the overall adaptation of the eye to the background luminance. Both Luminance Contrast and the minimum threshold have an impact on visibility and under observation, provides an explanation of the cause and nature of Glare. Lighting source spectral distribution effects perception, performance and preference and is extremely complex due to the inherent subtlety of the visual system. 3

4 The Color Nature of Light SPD curves show the precise color output of a light source across the visible spectrum. When considered along with color temperature and CRI a complete indication of the color composition of the light source can be evaluated. 4

5 SPD ( Spectral Power Distribution) Provides a visual profile of the color characteristics of a light source. 5

6 Source Flux (lm) Intensity (cd) Interpret ter Illuminance (fc) Modifier Perception p of Color Reflectance 6

7 COLOR TEMPERATURE Describes the perceived color appearance of a light source. The absolute temperature of a black body radiator having a chromaticity equal to that of the light source (it is implied that t the source is an incandescent glowing body). 7 Below 3300K Warm (red through yellows) 3300K K Neutral (white) Above 4000K Cool (blue-white) Object color is dependent on the spectrum of the light source. Good and bad. IES Handbook states Pupil size and subjective judgments of brightness are principally dependant on lamp color rather than measured illuminance i More blue in the source the brighter it will appear.

8 CIE CHROMATICITY DIAGRAM It is found that many different combinations of light wavelengths can produce the same perception of color. This can be put in perspective with the CIE chromaticity diagram. Describes the perceived color nature of a light source. No unit, a numeric expression X-Y chromaticity coordinates. Any color of light can be created by mixing proportions of the additive primaries red, blue, green. 8

9 BLACK BODY LOCUS Allows the plotting of color temperature referenced to the black body. Absolute temperature ( in Kelvin) of a black body radiator and located as to its X-Y chromaticity coordinates. 9

10 10 CRI ( Color Rendering Index ) An international system used to rate a lamp s ability to render object colors. The higher the CRI, (based on a scale), the better colors appear. CRI ratings of various lamps may be compared, but a numerical comparison is only valid if the lamps are also rated for the same chromaticity. CIE standard test colors. The CRI rating of a specific lamp is determined by testing eight standard color samples that are illuminated by a reference light source of similar color temperature defined as having a CRI of 100 Eight (8) pastel colors evaluated to the General Color Rendering Index (an average). Four (4) saturated colors, one (1) approximating a Caucasian complexion one (1) of olive green tone all evaluated to the Special Color Rendering Index.

11 Fovea Human Eye - Detail Section Blind Spot Retina Receptive Fields Rods Cones Ganglion? Optic Nerve 11

12 The "Standard Observer" The "Standard Observer" or "the" photopic sensitivity functions comes from CIE and dates to This data represents the sensitivity of the 2 degree visual field associated with foveal vision. The fovea is the central area of the retina, also called the macula, extending about two degrees out from the center of the eye. This central area is filled with cone photoreceptors although the fovea contains fewer short (blue) cones and no rods at all. Foveal vision uses this area of the retina, and the response to radiation is described d by the photopic sensitivity function Para-foveal describes the region surrounding the fovea, corresponding to the retinal area from two to ten degrees off-center. The response of the combined foveal and para-foveal regions to radiation is described by the variant of the photopic sensitivity function known as the CIE Supplementary Observer. This region of the retina has a mix of photoreceptors, with all three types of cones and rods present. Supplementary Observer" sensitivity function corresponds to the 10 degree visual field that is described as para-fovea. Peripheral refers to the region of the retina outside the central ten degree area. The periphery of the retina has a low density of cones of all three types, but is dominated by rods. Foveal, para-foveal and peripheral vision work together to produce our entire visual perception, but each has distinct characteristics 12

13 Human Eye - Detail Section 13 By population, about 64% of the cones are red-sensitive, about 32% green sensitive, and about 2% are blue sensitive. The "blue" cones have the highest sensitivity and are mostly found outside the fovea.. There are fewer blue cones, but the blue sensitivity is comparable to the others, so there must be some boosting mechanism. In the final visual perception, the three types seem to be comparable, but the detailed process of achieving this is not known.

14 Colour vision is provided by the cones, of which there are three distinct classes each containing a different photosensitive pigment. The three pigments have maximum absorptions at about 420, 530, and 560 nm and the cones are often called blue, green, and red. The cones are not named after the appearance of the cone pigments but are named after the colour of light to which the cones are optimally sensitive. The use of short-, medium-, and long-wavelength cones is a more logical nomenclature. The existence of three spectral sensitivity functions provides a basis for colour vision since light of each wavelength will give rise to a unique ratio of short-, medium-, and longwavelength cone responses. The cones therefore provide us with colour vision (photopic vision) that can distinguish remarkably fine wavelength changes. Colour Vision 14

15 Photopic Vision Color vision under normal lighting conditions during the day. In the range above 3.4 cd/m2 Mesopic Vision The combination of the higher total sensitivity of the rods in the eye for the blue range with the color perception p through the cones results in a very strong appearance of blueish colors Scotopic Vision Vision "in the dark", below.034 lm/m2. In that range, the human eye uses rods to sense light. The rods have a single absorption maximum of about 1700 lumens/w at a wavelength of 507 nm Scotopic vision is color blind. The sensitivity range of the rods makes the eye more sensitive to blue light at night Visual Response Luminance (Log cd/m 2 ) Pupil Diameter (mm) Retinal Illuminance Moon Light Indoor Lighting Sun light Scotopic Mesopic Photoptic No Vision Poor Acuity Scotopic Threshold Photopic Threshold Street lighting Levels Rod Saturation ti Best Acuity Begins Good Colour Vision Good Acuity Possible Damage The human eye responds to the visible radiation in a way that is depended on the luminance levels. 15

16 The Purkinje Shift The visual system is not equally sensitive to all wavelengths. As the eye adapts to various luminance levels its spectral sensitivity changes, and is determined by a combination of the Rod and Cone response curves. Scotopic spectral sensitivity is much higher than photopic sensitivity at all but the longest wavelengths, and peaks at approx. 500 nm. The shift in peak sensitivity has a perceptual consequence, namely the Purkinje shift. In photopic conditions, wavelengths near 550 nm appear to be brighter than those near 500 nm. The reverse is true in scotopic conditions: wavelengths near 500 nm are brighter than those near 550 nm. You can notice this phenomenon by comparing yellow and green objects. During the day, the yellow object will appear lighter, but as the sun goes down (and the scotopic system begins to operate) the green object will appear lighter. 16

17 Luminous Efficiency Functions 17 Purkinje Shift The shift in wavelength distribution of sensitivity from cone (daylight) vision to rod (night) vision. Peak Daytime (photopic) green light 555 nm. Peak night time (scotopic) blue-green light 507 nm. The human eye responds to the visible radiation in a way that is depended on the luminance levels. This means that if the variation of the luminance is such that the overall luminance remains in the photopic range, then the SLEF remains constant i.e. the V(λ). Similarly, when the luminance is in the scotopic range, but the SLEF will be the scotopic one V'(λ). Light meters and photometric devices are generally calibrated to the cone spectral sensitivity known as the photopic response. The additive response of the rods and cones varies with the adaptation level within the Mesoptic range resulting in a range of SLEF curves throughout the range.

18 Contrast detection is the basic task from which all other visual behaviors are derived." Illuminance criteria have been proven to be inadequate predictors of the effectiveness of lighting systems. Although the visibility of targets is typically directly proportional to illuminance (all other variables held constant), there are too many intervening variables that determine the visual stimulus and the efficiency with which that stimulus is processed by the visual system." Each critical detail of a seeing task must differ in brightness or colour from the surrounding background in order to be seen. Visibility is at a maximum when the luminance contrast (and color contrast if present) of details with the background is greatest. Contrast is far more important than absolute light. If an object's contrast is too low to be seen, then other visual factors are irrelevant. 18

19 Method Comparisons Illuminance: Luminance: Delivers Light To Road - Broad brush approach Pavement is uniformly visible to driver - Driver confidence STV: Objects are visible - Driver and pedestrian Safety Roadway Standard Practice Committee is studying a replacement metric for STV. 19

20 Small Target Visibility (STV) Determines the visibility level of an array of targets with reference to a driver: Luminance of the targets Luminance of immediate background Adaptation ti level l of adjacent surroundings Disability glare (Veiling Luminance) 20

21 Small Target Visibility V L = Visibility Level VL for a target at a point for one viewing point and direction is the amount above the visibility ibilit threshold h as seen by the observer. 21

22 22

23 Berman s Application Sources with scotopicaliy richer spectral content need less photopic luminance to enable a given level of visual performance, visual clarity, and brightness perception. An examination of currently available lamps, suggests that t there is a substantial opportunity to increase lighting energy efficiency in a highly cost-effective manner by considering lamp spectrum 23

24 Source Efficiency The S/P ratio of a light source is determined d by measuring the output in Lumens or Lux using a light meter or spectrometer calibrated first to the Photopic vision sensitivity curve, and then measuring the same light with instruments t calibrated to the Scotopic vision i sensitivity curve. The resulting numbers form a ratio that is expressed as a single number. S/P Ratios for Common Sources 24

25 Pupil Size Pupil size is important in lighting applications because it affects visual acuity and depth of field. Visual acuity is the ability to resolve fine detail Depth of field is the ability to maintain objects in good focus over a range of object distances (the range of distance is defined as the depth of field). The improved p acuity y more than offsets the decrease in retinal Illuminance that accompanies p the smaller pupil size. Short Depth of Field 25 Great Depth of Field

26 Mesopic vision Mesopic vision is more complicated than photopic or scotopic vision. Visual performance will depend greatly on whether objects lie in the sightline and cast images on the cone-dominated fovea or in roddominated peripheral vision. The fovea contains mostly cones, so viewers will continue act as if they are using photopic vision for small, fixated objects. The result is relatively good acuity and photopic color vision. The spectral sensitivity will be photopic with no Purkinje shift to low wavelengths. Objects viewed in peripheral vision will be seen in a mixed photopic/scotopic vision. As distance from the fovea increases, the balance shifts increasingly toward scotopic vision. Acuity is poor, color vision absent and spectral sensitivity shifted to lower wavelengths. The balance also shifts more toward scotopic vision of light level declines from the top of the mesopic range toward the bottom. 26

27 MOVE (Mesopic Optimization of Visual Efficiency) There are definite needs for a practical system of mesopic photometry to be used in lighting dimensioning and measurement of road and other outdoor lighting applications A research consortium, MOVE, established a basis for performance based mesopic photometry. The work steered away from conventional techniques, where only one aspect of visual performance had been considered at a time, and developed a method addressing the task of night-time driving. The driving task was divided into three visual subtasks, which are characterized by the questions: CAN AN OBJECT BE SEEN?, HOW QUICKLY?, WHAT IS IT? These visual tasks are related to the detection of a visual target, the speed of detection and the identification of the details of that target 27

28 28 Mesopic Models Instead of trying to describe the detailed performance of the eye under a given set of conditions, the emphasis has been on developing a system which can be readily implemented in practice Both the model of MOVE and the recently proposed X- model by Rea and others, are linear descriptions of mesopic luminous efficiency between the scotopic V (λ) and the photopic V(λ) functions. The models must to V(λ) at the upper end of the mesopic region and to V (λ) V(λ) a the lower end. Thus the practical model of mesopic photometry takes the form V mes (λ) = x V(λ) + (1 - x) V (λ), with x being a function of the level of illumination.

29 ASSIST Proposed Unified Luminance Table Based on the modified MOVE model 1. Pick s/p ratio of source 2. Pick Photopic Luminance 3. Read Unified Luminance 29

30 IES Position Paper More research required With respect to HPS and MH lamps, for two installations providing equal (photopic) light levels at night, peripheral (off-axis) detection of objects is better under the MH lamp, foveal (on-axis reading a sign) detection would be equal under each lamp. HPS lamps are rated slightly more energy efficient than MH lamps MH lamps might be more energy efficient at providing off axis visibility. HPS lamps will always be more energy efficient than MH lamps for providing on-axis visibility Nema Postion Paper Published 30 Similar Position as IES

31 Preference It cannot be assumed that light sources that produce a perception of greater brightness and greater clarity, and/or better task performance will be preferred for functional lighting. Preference is based not just on the perception of the lighting and the ease of task performance In the space, but also on the context of the space and people s expectations for the lighting of such a space 31

32 Thank You

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