light levels classact srs enabled. in this presentation you will: explore the measurement of the...

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Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination and distance

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Page 1: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

ClassAct SRS enabled.

In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

and distance

Page 2: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

In this presentation you will learn about light, how it can be detected, how its luminous intensity can be measured, and how distance affects the illuminance on a surface.

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Page 3: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Each color has its own specific wavelength, ranging from violet with the shortest wavelength (400 nanometers) to red with the longest wavelength (700 nanometers). All these colors are part of the visible spectrum.

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White light actually consists of numerous colors, which can be separated by passing white light through a glass prism. This causes the light to refract and disperse.

The Visible Spectrum

Page 4: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

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The Electromagnetic Spectrum

The visible spectrum is one very small part of the electromagnetic (EM) spectrum, which covers an enormous range of frequencies.

106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018

100m

10m

1m 10cm

1cm

1000µ

m1m

m

100µ

m

10µm

100nm

10nm

1nm

0.1nm

1000m

1000nm

1µm

X-ra

ys

Ultra

viole

t

Visib

le

Infra

-red

Micro

-w

ave

s

Frequency in Hz

Wavelength700

nm

600nm

500nm

400nm

Ra

dio

, tv

Lo

ng

- w

ave

s

Page 5: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

The Human Eye

The quantity of light entering the eye is controlled by the size of the pupil, but it is cells in the retina that detect the amount of light and the color.

The retina consists of two types of light-sensitive (photoreceptor) cells called rods and cones.

These cells react to the different light frequencies, producing different photochemical reactions that send electrical signals to the brain.

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The human eye can only detect frequencies in the visible spectrum.

Pupil

Retina

Page 6: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

The Human Eye

Rods can distinguish between different intensities of light, but not between light of different frequencies…

… whereas cones allow us to see color.

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Nucleus

Outer segment of cone cell containing

photosensitive chemicals

Cone Rod

Nucleus

Outer segment of rod cell containing photosensitive chemicals

Page 7: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

1

What is the name given for the part of the electromagnetic spectrum in which light can be seen by the human eye?

Question

A) Magnetic spectrum

B) Visible spectrum

C) Frequency spectrum

D) Invisible spectrum

Page 8: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

2

Rods and Cones are light-sensitive cells in the eye that allow us to see light intensity and color. Which of these two types of cells detects color?

Question

A) Rods

B) Cones

Page 9: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Crookes’ Radiometer

A radiometer consists of a set of vanes, each shiny on one side and blackened on the other.

When exposed to light, the vanes revolve. The more light, the faster the vanes revolve.

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One instrument that can demonstrate the energy emitted by a light source is Crookes’ radiometer.

It is difficult to measure light energy with this instrument, as the vanes can move very fast and do not respond quickly to changes.

Vanes

The vanes are mounted on a spindle in a vacuum.

Page 10: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Illuminance in lux

Res

ista

nce

in

Photoresistor

A photoresistor, also called a Light Dependent Resistor (LDR), is a sensor that reacts to the quantity of light falling on it by changing its resistance.

The resistance of a photoresistor usually decreases as the ambient light falling on it increases.

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Modern electronics has given us several light sensing devices.

Page 11: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Photodiodes & Phototransistors

Photodiodes and phototransistors are light-sensitive semiconductor devices.

They can be used in a variety of sensor circuits that detect the presence and the amount of light.

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Each of these devices reacts to light by controlling the flow of electric current passing through it.

Page 12: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Photometry

Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye.

The human eye is not equally sensitive to all wavelengths of light.

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In this part of the presentation you will learn about 4 terms that are commonly used in photometry:

•Luminous Intensity•Luminous Flux •Luminance• Illuminance

Photometry attempts to account for this by weighting the measured power at each wavelength with a factor that represents how sensitive the eye is at that wavelength.

Page 13: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Luminous Intensity

Luminous intensity (I) is a measure of the power emitted by a light source in a particular direction.

The unit of luminous intensity is the candela (cd) from the Latin word for “candle”.

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Page 14: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Luminance

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Luminance (L) is the luminous intensity emitted by a unit area of a light source.

The unit of luminance is the candela per square meter (cd/m2).

Luminance is often used to measure light emission per unit area from flat, diffuse surfaces, such as video displays.

The luminance is a measure of how bright the surface will appear to the eye from a particular angle of view.

Page 15: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Luminous Flux

Luminous flux (Ф) or luminous power is a measure of the power emitted by a light source in all directions.

The unit of luminous flux is the lumen (lm), from the Latin word for “light”.

Luminous flux is often used to measure the useful power emitted by a light bulb.

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For example, a typical 100 watt incandescent light bulb emits about 1700 lumens.

When comparing different types of light bulbs, energy conscious consumers compare the luminous flux of each type of bulb.

Roughly the same amount of light can be produced by a 25 watt compact fluorescent light bulb at a quarter the electricity cost.

Page 16: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Illuminance

Illuminance (E) or illumination is the total luminous flux falling on a unit area of a surface. It is a measure of how brightly the surface is lit.

The unit of illuminance is the lux (lx), which is the same as lumens per square meter (lm/m2).

If 1 lumen (lm) of luminous flux uniformly lights a surface of area 1 m2, the illuminance of the surface will be 1 lux.

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Some typical illuminance values in your everyday surroundings are:Bright sunlight 50,000 – 100, 000 luxSunset 1 – 100 luxFull moonlight 0.01 – 0.001 luxClassroom lights 100 – 300 lux

1 m2

1 lumen

Illuminance = 1 lux

Page 17: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Area=100m2

= 1 lux

100 lm

Lux vs Lumen

For example: 100 lumens, concentrated into an area of one square meter, will light up that square meter with an illuminance of 100 lux.

The same 100 lumens, spread out over a hundred square meters, will produce a dimmer illuminance of only 1 lux.

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The difference between the lux and the lumen, is that the lux takes into account the area over which the luminous flux is spread.

Area=1m2

= 100 lux

100 lm

Page 18: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

3

Luminous flux of 100 lumens falls on a flat surface of area 20 m2. What is the illuminance of the surface?

Question

A) 100 lux

B) 20 lux

C) 5 lux

D) 1 lux

Page 19: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

4

What is illuminance measured in?

Question

A) Candela

B) Lumen

C) Candela per square meter

D) Lux

Page 20: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Effect of Distance on Illuminance

Investigation of illuminance shows that it obeys an Inverse-Square Law, which states that…

“… as a light source is moved away from a surface, the illuminance decreases by a factor that is inversely proportional to the square of the distance”.

Thus the illuminance of a surface by a source of light 2 m away is 1/4 of the illuminance at 1 m from the source.

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This can be written as:

r2r

3r4r

5r

I 1

r2

Page 21: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

Effect of Distance on Illuminance

Conversely, for two light sources, one at 1 m from a surface and the other at 2 m, to give the same illuminance to the surface,…

…it would be necessary for the source at 2 m to be 4 times as bright as the source at 1 m.

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1m

2m

Page 22: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

5

If you double the distance between a light source and a sensor, how is the illuminance affected?

Question

A) The illuminance quarters.

B) The illuminance halves.

C) The illuminance remains the same.

D) The illuminance doubles.

Page 23: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

6

If the illuminance at a distance r from a light source is 400 lux, what would the illuminance be at a distance of 2r?

Question

A) 800 lux

B) 200 lux

C) 100 lux

D) 1 lux

Page 24: Light Levels ClassAct SRS enabled. In this presentation you will: explore the measurement of the quantity of light explore the relationship between illumination

Light Levels

After completing this presentation you should be able to:

Summary

show knowledge of some of the sensing elements used for the measurement of light.

show knowledge of some of the units used in the measurement of light.

show knowledge and understanding of the Inverse-Square Law for illuminance and distance.

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