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Vaasa University of Applied Sciences Principles of Modern Physics Laboratory of Physics I-IT-2N2

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Basic of Photometry

Student Name: Binh Bui

Student Number: e1300518

Group: I-IT-2N2

ContentsI.  Physical Background ................................................................................................... 2

II.  Measuring Methods ..................................................................................................... 2

1. The Luminous Efficiency: ........................................................................................... 2

2. The Reflectivity: ........................................................................................................... 2

III.  Results: ..................................................................................................................... 3IV. Comparation .................................................................................................................. 6

1. The Luminous Efficiency: ........................................................................................... 6

2. The Reflectivity: ........................................................................................................... 6

Picture of Lab Record Sheet .............................................................................................. 8

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I. Physical Background

In this laboratory exercise, we are going to do experiment on the topic of photometry, which

is the science of measurement of light. It is different from radiometry, which is the

measurement of radiant energy and concerned with electromagnetic radiation as a form of

energy. Photometry is essential due to it analyzes perceived brightness to human eye,evaluates radiation in respect of its usefulness in human vision.

This work aims to give a general conception of photometry, as well as familiarizes the

student with the definition of basic photometric quantities and their SI units. We will take turn

to measure the illuminance E produced by a LED lamp with different directions, thus, to get

the luminous efficiency K of the light source. Finally, the reflectivity p is also obtained in this

exercise by measuring the values of illuminance entering the surface and luminance, A,

emerging from the surface.

II. Measuring Methods

We are going to use:

-Lux-meter

-LED lamp

-510 colored cardboards

-Light source

-A ruler and a protractor

1. The Luminous Efficiency:

•  Connect a 230 V (rms) voltage across a LED lamp•  Place the sensor of the lux-meter at 0.5m from the bulb

•  Measure the value of voltage across the lamp, ΔJ, and the current through it , with

two multimeters. Multiply these with each other to find out the electric input power of

the light source, >. Read the angle corresponding to the telescope’s orientation as

precisely as possible.

•  Using the rotating stand of the lamp, measure the values of 2 in 10° steps from 0°

4the head of the lamp directly towards the meter) to 180°

Since we can assume that the light produced by the source is symmetrically distributed

around it. We can omit180° to 360°.

2. The Reflectivity:•  Place the paper sample package onto a horizontal surface, which is well illuminated

by the lamps on the ceiling

•  Measure the magnitude of 2 incident on the package.

•  Attach a metal bar vertically to a rigid stand, and fasten the luminance sensor 4the

black metallic cylinder) to the vertical bar so that it is directed toward the table at an

angle of about 45°.

•  Open the paper sample package at a chosen page, and place it so that the cylindrical

sensor measures light emitted from the paper sample. Record the value of A for the

sample.

•  Calculate the magnitude of p by using this equation: = 

 

•  Repeat the sequence for the other samples, too.

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 As following the instructions, we were able to observe the spectral lines and obtain all the

necessary values:

-Voltage and Current across the LED lamp

-Values of illuminance E

 All the values will be manifested below in part III.

III. Results:

Prerequisite quantities Magnitude

V 230V AC

I 9.12mA

r 0.5m

With V and I, we are able to obtain the electric input power P of light source:

=  . = 230 ∗ 9.12 ≈ 2.1 

Here is the table for the illuminance E measured by lux-meter:

Angle E

0° 658.40 lux

10° 447.10 lux

20° 67.26 lux

30° 41.14 lux

40° 36.21 lux

50° 26.08 lux

60° 21.85 lux

70° 19.90 lux

80° 12.84 lux

90° 9.73 lux

100° 5.53 lux

110° 5.70 lux

120° 3.70 lux

130° 2.28 lux

140° 1.70 lux

150° 1.20 lux

160° 1.00 lux

170° 0.90 lux

180° 0.65 lux

Give the measured values of E,  and r to

the Mathcad document ‘photometry.mcd’,

and let the software calculate the values

of luminous intensity as a function of the

direction angle, ;. The document will plot

the corresponding Iv() curve for the light

source. This shows you how the studied

source emits light to its surroundings.

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From the Mathcad document, we were able to obtain the total luminous flux:

It expresses the power at which the light source produces visible light. Also, the luminous

efficiency, K, of the light source:

Here is the table for the quantities needed to calculate the coefficient of reflectivity, L and E:

Color L

Red 1.4 cd/m2 

White 2.5 cd/m2 

Yellow 2.9 cd/m2 

Light Green 1.2 cd/m2 

Dark Green 1.1 cd/m2 

Black 0.5 cd/m

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IV.Comparation

1. The Luminous Efficiency:Use the data in Table 1 to calculate the luminous efficiency of one type of a fluorescent

lamp; for this you do not need to do any measurements. Compare the experimentally found

value with the calculated one.

I will take the Bright White 2X as it’s familiar with what we practice with in the lab which has

total luminous flux:

= 580  

 And P:

= 15  

Thus, luminous efficiency will be:

=

=580

15   ≈ 38.67

 

 As we can see, the result is close to the value we measure earlier:

= 31.683

 

2. The Reflectivity:

Compare the apparent brightness and the reflectivity of the paper samples. Do they match oris it difficult to understand the results?

The results were easy to understand as we can see from real experiment is that, for the

yellow one which has the highest coefficient of reflectivity, tends to shines a lot brighter,

meaning it absorbs less than the other, while on the other hand, the color black seems to

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has the highest absorption among the cardboards we tested.

Source: http://www.cns.nyu.edu/

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Picture of Lab Record Sheet

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