13. measure light p5

measure light p5

1. MEASURE LIGHT Ms. DIAH

2. What is the speed of light ? D o you know what is the fastest thing ever? The cheetah? From the animal kingdom? Sure! Usain bolt ? In humans, yep! But do you know what is faster than the fastest? Look around you! Any guesses? It is… LIGHT! HOW FAST IS THE SPEED OF LIGHT ? m athematically , light travels at the speed of 300000000 meters per second in vacuum OR 3 . 10^8 m/s . It is a constant quantity which remains the same throughout the reaches of humans. This constant attribute, though changes when the medium changes. In vacuum, there is no obstacle for the light waves and hence, it travels unobstructed.

3. Speed of Light Vacuum: 3 x10 ^8 meters per second. Water: 2.26 x10 ^8 meters per second. Air: 2.9 x10 ^8 meters per second. Glass: 1.9 x10 ^8 meters per second.

4. Our eyes can tell the difference between bright light and dull light. To measure light accurately , we can use a light meter The unit that we use to measure the brightness (or illuminance) of light is the LUX (lx) This measures the intensity of the light as it appears to our eyes. It does not measure the energy of the light source Can we Measure Light ?

5. Light Intensity L ight intensity (quantity) is measured in foot-candles in the United States, or in lux in most other countries. Even in the United States lux is replacing foot-candles. As we've noted, a foot-candle equals about 10.74 lux (or, for a rough conversion, multiply foot-candles by 10 to get lux). To provide some points of reference:   sunlight on an average day ranges from 32,000 to 100,000 lux TV and film sets are lit at about 600 lux a bright office has about 400 lux moonlight represents about 1 lux starlight measures a mere 0.00005 lux

6. Use information in the table Time of day or place Lux (lx) measurement Direct sunlight 32000 - 130000 Full daylight 10000 - 25000 Shady place on a cloudy day 11500 Overcast day 1000 Sunrise or sunset 400 Inside an office 400 Full moon on a clear night 00 starlight 0,0011

7. Light Meter L ight meters are used to measure light intensity.

8. Light Meter As we've noted, our eyes are drawn to light areas in a scene as illustrated in this painting. Therefore, you can use light to emphasize the scene's center of interest and downplay secondary or potentially distracting elements. Before you can use relative intensities creatively (and avoid intensity related problems) you need to be able to accurately measure light intensity. B ecause the eye is a rather unreliable judge in setting up lighting, you need to use either a light meter or a keen eye coupled with a high-quality color monitor. Although the latter is preferred for making final adjustments, when lights are first being put into place it's much faster to use a light meter. There are two types of meters: reflected and incident.   

9. How to Measure Light Intensity how to measure light intensity, the difference is between lumens and illuminance (and what they mean )

10. What is the Best Metric to Measure Light Intensity ? Illuminance is the metric that is used to measure the light intensity within a space. It is measured in footcandles or lux – it is the amount of light (lumens) falling on a surface (over any given square foot or square meter). Therefore, light intensity is measured in terms of lumens per square foot ( footcandles ) or lumens per square meter (lux). Measuring the amount of light that falls on a surface allows us to evaluate if we have sufficient light to perform various visual tasks. 

11. What are Lumens ? Lumens (lm) are the unit of measurement we use to quantify the amount of visible light the human eye can see. The luminous flux of a particular light source is measured in lumens. You many have noticed  when you buy light bulbs for your house that they indicate lumen output. The higher the lumen output, the ‘brighter’ or higher intensity the light source; the lower the lumen output, the less bright or lower intensity the light source. When you purchase light bulbs based on their intensity or brightness, it is lumens you‘re after, not watts – watts simply determine a bulb’s energy consumption. By understanding lumens, we can explore other lighting metrics such as illuminance ( footcandles /lux) and how this plays a key role in assessing the intensity of a light source.

12. What is Lux ? Lux is simply the unit of measure used to describe the number of lumens falling on a square foot ( footcandles ) or square meter (lux) of a surface. So let’s say you have a light source with 1,000 lumens. If all of those 1,000 lumens are spread over a surface area of 1 square meter, you’d have an illuminance of 1,000 lux – i.e. the brightness of an overcast day. But what if we spread this over 10x the area i.e. 10 square meters? Well, the illuminance or lux would decrease to a less intense and dimmer 100 lux. We use the same approach for footcandles , only our units are lumens per square foot. The reason we measure light intensity is to ensure a certain “standard” of illumination is met. it makes a valuable difference for a photographer (whose work is specifically centered around light) as it would in surgical theater or other indoor environments such as offices.

13. What is a Footcandle ? A footcandle is a measure of light intensity – it’s the number of lumen per square foot. Now, you might be thinking we already covered lux, so why add in this metric? Different people use different metrics and for different reasons. Simply put, where 1 lux equals 1 lumen in a square meter, 1 footcandle equals one lumen per square foot. 

14. What is Luminous Flux ? Luminous flux is how to measure the perceived power or total amount of light output from a light source. When the number of lumens – the unit-amount of visible light a human eye can see, is used to measure the intensity of a light source. A one-meter sq. surface area is required (lux) to determine the luminous flux value.

15. How to Measure Light Intensity Using a Light Meter 1. Measure Ambient Light In the Room To begin, turn off any lighting in the room you’re about to measure. Turn on the light meter to establish what’s known as the baseline measurement – the ambient light. This means you’re able to see how much the existing lighting adds to the room once the lights are then turned on .   2. Turn On the Lights, Take Your Measurement   From a central area of the space, ensure your light meter is set up to record your new reading. Remember not to rush – allow the lighting a few moments to reach full brightness (especially if you’re measuring light from CFLs). 3 . Note Your Differential Reading   Simply subtract the ambient light level from the illuminated level – known as the differential (or delta) measurement, this is the amount of light the existing luminaires produce. With this light measurement unit, you can asses how it compares to the optimal lighting levels required.   4 . Check Other Areas of a Room   For open office lighting or a corridor, the reading you get from your light meter should, in theory, be consistent. However, it might be worth checking any potential “blind” spots just to be sure you have consistency.

16. To measure quantity of light from a source Historically, a lumen was the amount of light from one candle which fell on one square foot of area, one foot away from the candle. Our historic “standard” candle produces 12.57 lumens A four foot fluorescent lamp produces about 3000 lumens Your standard 60 Watt light bulb (incandescent) produces about 900 lumens One foot One lumen of light falls on a surface one foot square, which is held one foot away from the candle.

17. To measure quantity of light striking of surface Lumens per square foot, other wise known as footcandles (fc) • or , Lumens per square meter otherwise known as Lux (lx) These are the common measurements made by a light meter One foot-candle = 10.76 lux

18. Remember , every lightning application have 3 element : The Source is all the hardware , This is what most people think of The as the lighting system, but just as important are … The Surface which is the room environment Source and whatever you are looking at The Purpose The reason you need the light, the activities or tasks that people are doing that require certain visual conditions