Light Workshop Introduction to Light, Basic Principles

Transcription

1 AD30400 Video Art Prof. Fabian Winkler Spring 2014 Light Workshop Introduction to Light, Basic Principles We still do not know what exactly light is. It appears immaterial to the human eye, yet it renders our world visible. This workshop investigates some of light s physical properties and introduces key terms that help us to establish an emerging vocabulary for the discussion of work in this class. Light Spectrum The ambiguity of light becomes already apparent in the description of what it actually is. On the one hand, physicists describe light as particle, the flux of mass-energy units emitted by a source of radiation, such as the sun or a candle. On the other hand, light is also understood as waves that differ in length and frequency. In this workshop, we will only look at light as a tiny part of the whole electromagnetic spectrum as we known it today (see diagram above). The human eye is susceptible to frequencies in the range from 380nm to 720nm, these are the boundaries for visible light. Within these limits, all visible colors are located - they are at their most intense at:

2 violet = 440nm blue = 480 nm green = 520nm yellow = 570nm red = 650nm The primary colors for the mixing of light (additive color mixing) are different from the primary pigment colors (yellow, blue, red - subtractive color mixing): Corresponding LEE filter: Corresponding ROSCO filter VIOLET BLUE 448nm No GREEN 518nm No. 124 (89?) ORANGE RED 617nm No You can get gels/filters online from Fullcompass Pro audio, Video, AV & Lighting ( It doesn t need to be exactly the right number for your first additive color mixing experiments, a good selection of red, green and blue theater gels is a good starting point. Additive Color Mixing Additive Colors are created by mixing spectral light in varying combinations. When all of the frequencies of visible light are radiated together the result is white (sun) light. Red, green and blue (RGB) are the primary stimuli for human color perception. In the RGB color model these three primary colors can be combined in 256 intensities each for a maximum of 256x256x256 = 16,777,216 possible color combinations. Subtractive Color Mixing Subtractive colors are seen when pigments in an object absorb certain wavelengths of white light while reflecting the rest. The wavelengths left in the reflected/transmitted light make up the color we see. Cyan, magenta and yellow (CMY, K stands for black) correspond roughly to the primary colors in art production. The CMYK model used in printing lays down overlapping layers of varying percentages of transparent cyan, magenta and yellow inks. Light is transmitted through the inks and reflects off the surface below them. In theory, the combination of cyan, magenta, and yellow at 100% creates black (all light is being absorbed). In practice, however, CMY usually cannot Fabian Winkler, Light Workshop, Basics, p.2

3 be used alone (imperfections of ink, etc.). To provide a genuine black, printers resort to adding black ink indicated as K. It is important to know that CMYK cannot reproduce the brightness of RGB colors. Color Temperature Light sources very often differ in the color temperature of the light that they produce. For example, candlelight looks more orange than the light produced by fluorescent lights. Light reflected off of snow looks more blue than the light during sunset, etc... The theory of color temperature is based on the fact that there is a fixed relationship between the temperature of an incandescent i body and the color of the light that it emits. This light color is defined as color temperature. ii It is measured in a scale using degrees Kelvin (K). Manufactured artificial lights are normally made to produce light of 3200K, which is the standard figure for artificial light, but there are some which produce 5500K, which is the standard figure for daylight. iii source: Lyver and Swainson: Basics of Video Lighting, p.4 The color temperature of a light source is determined by comparing the color of light that it emits with that emitted by a comparative radiator. The comparative body absorbs any outside radiation that strikes it and is called a black body - also known as a Planck radiator. It is heated until it shows the same color as the light source. This temperature is called color temperature, and is measured in Kelvin degrees. iv Luminous Flux Luminous Flux is the luminous power of the light source for light emitted in all directions (see figure 149). Unit: lumen (lm). Luminous flux is very often given for data projectors e.g lumens, etc... Luminous Intensity Luminous Intensity is the unit of light emitted in a specific direction (see figure 151). Unit: candela (cd). Luminous intensity is one of the specifications for LEDs, e.g. very bright, (high-brightness) LEDs have a luminous intensity of more than 3000mcd (microcandela). Fabian Winkler, Light Workshop, Basics, p.3

4 Illuminance Illuminance is the illumination of a given area at a given level. It is measured in lux (lx). One lux is the amount of light falling on an area of one square meter, one meter away from the light source of one lumen. We see the word lux very often in the description of low light features of video cameras (e.g. 0.1 lux super night shot function). If one candela source radiates uniformly in all directions, the amount of luminous flux falling within a solid angle is called a lumen. Most manufacturers will quote their bulbs with as having a certain number of lumens per Watt. This gives us an indication of how bright one bulb is against another. For example, a xenon lamp is brighter than a standard thungsten lamp of the same power v Power refers to the number of Watts of the lamp, based on Ohm s law: P = E x I. For instance, a 60W light bulb connected to 120V AC consumes 0.5A current. Fabian Winkler, Light Workshop, Basics, p.4

5 Light and Shadow In very simplified terms, one can say that light travels in straight lines from the light source to the surface that it illuminates. But there are also many structures which light cannot penetrate. If some of these structures are between light source and destination area, some of the light beams are interrupted and don t reach their destination. These holes in the light are shadows. We distinguish between three types of shadows (see also the davinci s sketch on the next page) - 1) cast shadow (e.g. I-K and L-M), 2) attached shadow (underneath the nose and the chin) and 3) shading (partial shadow, deficiency in light, e.g. D-E). In his book Shadows and Enlightenment, Michael Baxandall further classifies these three types of shadows: In the case of the first sort of shadow, that which is caused by a solid intervening between a surface and the light source (as by a nose preventing light from reaching an upper lip (fig. 2)) the term projected shadow will be used; and when a projected shadow is thrown on a differentiable surface, it may still surely be described as cast. In the case of the second sort of shadow, on surfaces which face away from the light (like the under part of the nose), the best term will be self-shadow, which is the term used in computer vision studies. As for shading, the word is much too generally current not to use, and if there is a danger of ambiguity it can be qualified as slant/tilt shading, slant being angle on the vertical axis and tilt being angle on the horizontal axis. vi Fabian Winkler, Light Workshop, Basics, p.5

6 Light Sources Light Sources vary in extension, from sources that may be considered point sources through various levels of extended source to a notionally quite non-directional source - assuming infinite reflections of light from ambient surfaces - called ambient light. Point-like sources produce the sharpest-edged shadow, perfect ambient light would produce none. Extended sources produce softer-edged shadows with a divide between umbra, the part masked from the whole area of the light source, and penumbra, the border zone masked from only a proportion of it. vii Remember that if you want to produce sharp shadows, the light source is of vital importance. The cheapest possible light source to create relatively sharp-edged shadows is a clear light bulb with only a very short filament. You can get these light bulbs at every home improvement store. Remember also that shadows are influenced by the distance between light source, cookie (the object you want to have cast a shadow) and the projection surface onto which the shadow needs to be cast. The closer the cookie to the light source and the further away the projection screen, the bigger the shadow and the more blurry the shadow s edges. The closer the cookie is to the projection surface, the smaller the shadow and the sharper the shadow s edges. Fabian Winkler, Light Workshop, Basics, p.6

7 Linnebach Projector A Linnebach Projector, also called Linnebach Lantern, is one of the simplest projection devices. You can build such a projector for only a few dollars yourself. Linnebach projectors are especially useful as effective low-cost light sources for shadow projection. Encyclopædia Britannica defines a Linnebach Projector as a theatrical lighting device by which silhouettes, colour, and broad outlines can be projected as part of the background scenery. Originally developed in the 19th century by the German lighting expert Adolf Linnebach, it is a concentrated-filament, highintensity lamp placed in a deep box painted black inside. Linnebach projectors can be also used as crude slide projectors - the slides need to be very large though and the projected image has a slightly soft definition (due to no lens). Below is a sketch of Fabian Winkler, Light Workshop, Basics, p.7

8 a commercial Linnebach Projector but it is very easy to build such a device yourself, too. Further Resources Keller, Max: Light Fantastic: the art and design of stage lighting, Munich/New York: Prestel, 1999: Light and Colour (p.25) Handling Light and Colour (p.35) Optics (p.67) Lamps and Light (p.75) Color Plates and Color Gels (p.99) Baxandall, Michael: Shadows and Enlightenment, New Haven & London: Yale University Press, From the preface: this book is a discussion of shadows and their part in our visual experience. More particularly, it juxtaposes modern with eighteenth-century notions about shadows with a view to benefiting from a tension between them. Miller, James Hull: Stage lighting in the boondocks: a layman's handbook of downto-earth methods of lighting theatricals with limited resources, Colorado Springs, CO: Meriwether Pub., Fabian Winkler, Light Workshop, Basics, p.8

9 Samuel van Hoogstraten, "The Shadow Dance" (engraving, 1675) A new media artwork that was inspired by this engraving is Rafael Lozano Hemmer s Body Movies, 2001: Endnotes i Lamps are divided into two groups according to the way in which their light is produced: incandescent lamps... and discharge lamps.... An incandescent lamp is a thermal radiator. A thungsten filament is heated to produce both thermal and luminous radiation. Only 5-10% of the electrical power consumption is converted into emitted light - the rest is heat! (Keller: Light Fantastic, p. 75) ii Keller, Max: Light Fantastic: the art and design of stage lighting, Munich/New York: Prestel, 1999, p. 27. iii Lyver, Des and Swainson, Graham: Basics of Video Lighting, Oxford, Auckland, Boston, Johannesburg, Melbourne, New Delhi: Focal Press, 1999, p.4 iv Keller: Light Fantastic: the art and design of stage lighting, p.27 v Lyver and Swainson: Basics of Video Lighting, p.24 vi Baxandall, Michael: Shadows and Enlightenment, New Haven & London: Yale University Press, 1995, p.4 vii ibid., p.5 Fabian Winkler, Light Workshop, Basics, p.9