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Monday, May 12, 2003
The color of red wine varies from type to type, year to year and even bottle. Because of this, and my particular relationship to color, I have been researching a method of unambigously describing the color of red wine. Phrases like "bluish-purple", "brick" and "tinged around the edges" mean little because they describe the perception and imagination of an individual observer and may not look that way to you. A set of numbers will be an objective description of the color.
Taste, on the other hand, is a subjective quality, and cannot be described numerically, because so much mental processing accompanies taste, smell and tactile sensations that comprise most of the wine experience. So far, only color and a few other physical properties (pH, density, alcohol content, viscosity, etc.) can be quantified.
I have observed the light that is transmitted through wine and the light that is absorbed by wine. These are represented by the tranmittance and absorbtion curves, respectively. The transmittance curve represents the light you see when you look through a glass at a light source. Red, orange and infrared light is transmitted at various intensities, and the rest of the light is blocked. This is no surprise, because red wine is ... red.
The absorbtion curve, taken from a diluted sample, shows that some light in the blue and green regions is transmitted, and that there is an absorbtion maxima at 510-520 nm. A reference states that this is due to phenolic compounds, which contribute to the flavor of the wine. The relative height of this peak is hypothisized elsewhere to be indicative of the age of the wine sample. I have tennatively found it to also change when wine is oxidized by exposure to air.
For describing wine color, the transmittance curve is more useful. The height of the line represents how much light gets through - that is, how much you see. A typical spectrum is low in the blue (400-500nm) and green (500-560nm) regions, sloping up through the yellow (560-600) and orange (600-640) regions and maximizing in the red (640-700) and ir (700+), then the wine will look warm red. Anothe spectrum may be low until the red region and slope sharply upward - the color will be decidedly cooler, perceptually 'bluer' because of the lack of yellow. There is not any blue light actually transmitted.
A simple set of numbers that describes the slope of the transmittance curve will effectively describe the color of the wine. I have identified seven parameters that describe the curve, wl(mid), t(mid), m(mid), Ti, Tr, Ty and Tgb. Wl(mid) is the wavelength of the midpoint of the rise in the transmittance curve, and is the same on all of the samples I have run, 645 nm. Since it is invariant (doesn't change) it can be eliminated from the parameters. Additionally, the blue-green light is always low (<1% T) so that too can be eliminated.
This leaves T(mid), the transmittance of the midpoint of the curve, and M(mid), the slope at that point. These together describe if the curve is sharp (bluer) or gradual (warmer) and how dark the color is. Tir, Tr and Ty are the infrared, red and yellow-orange componants. Infrared is, of course, invisible, but indicates the total darkenss of the color as does T(mid). The ratio of Tr to Ty will determine the warmth, similar to the value of M(mid). Comparing these parameters will give an indication of the internal consistance of these measurements.
I'm running short on time right now (I need to get to work in 40 minutes) but this evening I will calculate the parameters for a few of the wine samples and compare them. The color of the wine should be describable by no more than two parameters.
The transmittance curve is superior to the absorbtion curve in assessing the color because 1) it represents the actual light seen by an observer looking through a sample of the wine, and 2) it can be performed on an undiluted wine sample, and so is not susceptable to experimental error.
Giant crater Gassendi (68 miles in diameter) lies on the north end of the 'Sea of Moisture'. A couple rilles can be made out inside the crater. At the bottom of the picture, a ray from crater cuts across the surface of the moon.
Picture taken with 6" f/4.9 Schwar dobsonian reflector, using a 9mm modified acromat eyepiece. Exposure was 1/15 of a second at f/3.1. Taken at 0900 UT 5/12/03.
Gibbous Moon in Infrared