# How do you measure illuminance (natural and artificial light)?

## The difference between Illuminance and Luminous Flux

Today in the lighting market there is a lot of confusion with technical parameters such as luminous flux (measured in lumens (Lm) and illuminance (measured in lux (Lx)). Most people pay attention to the luminous flux (Lm – indicated on the packaging of each LED lamp), but not the requirements for illumination.Most often, in the calculation takes the total luminous flux of the lamp or LEDs, without light and heat loss.

Light flux can only be measured in a special laboratory, do it yourself with improvised devices is impossible! In the regulatory documents there is a concept of luminous flux, but there are no specific requirements for it.

Illuminance can be measured by anyone without complicated equipment.What is illuminance?

Illuminance is the ratio between the light flux and the area on which it falls. And it must fall on this plane perpendicular. It is measured in lux (lx).

## Why do you measure the illumination?

Scientists have proved that bad (or, conversely, too good) light through the retina affects the workings of our brain.

As a consequence light affects the psychological state of a person: if there is not enough light – he feels depressed, reduced efficiency, sleepiness, if the light is too bright, it helps to excite, connect additional resources of the body, causing their increased wear and tear. Both – equally harmful.

If, however, the light is chosen correctly, then by improving the lighting, productivity in the workplace can be increased by 25-30%.

## Parameters for assessing lighting

Light waves as a type of electromagnetic waves are distinguished by the length and frequency of oscillations, which are linked to each other by the following mathematical relationship: b = c/&

where A, – wavelength; m;s – speed of propagation of light, 300 000 km/h; frequency of oscillation, Hz (1 Hz equals one oscillation in 1 s). Light intensity is measured in candela (cd). One cd corresponds to 60 U60 of light intensity, emitted perpendicularly from the surface of an absolute black body of 1 cm2 at a platinum solidification temperature of 1760 °С.

Illuminance is measured in lux. Lux (lx) is the illuminance of a surface, to each square meter of which light flux equals one lumen (lm) falls: 1 lux = 1 lm/1 m2.

Lumen is the luminous flux emitted within a solid angle of 1 stm by a light source of intensity equal to 1sv, expressed as the ratio of the illuminated area to the square of the distance from the light source. If a surface is illuminated by several sources creating on it illuminations ?,,E2, etc., then the total illuminance of the surface E will be equal to their sum.

Ripple coefficient. Changing lighting conditions of rooms causes adaptation of visual organs, which is based on physiological and photochemical processes that lead to changes in visual sensitivity. Frequent and abrupt changes in lighting conditions are reflected in the physical condition of the human body.

The speed of distinguishing and the stability of clear vision of objects also depend on the level of illumination. The speed of discrimination is especially high at illumination levels of 400-500 lux, the stability of clear vision corresponds to illumination levels of 130-150 lux.

Important factors to be taken into account in determining the illuminance of rooms are the color of interiors and the difference in brightness between the observed object and the background against which the object is viewed. Thus, brightness contrast depends on the level of illumination: the lower the illumination, the greater the contrast should be. The brightness of the background is determined by the amount of reflected light perceived by the human eye.

## Types of lighting

Illumination is provided by the arrangement of windows and the installation of fixtures.

In some cases, you need uniform lighting of the room, in others – the normative should be light workplaces, and lighting the entire room can be two or three times less. This depends on the purpose of space and achieved by using certain types of lighting fixtures and their location, which is provided by the project. Lighting can be natural or artificial.

## Natural light

Sources of natural light are:

• the sun,
• the moon (more precisely the light reflected by it),
• the diffused light of the firmament (this is not just a poetic name, a term used in protocols for measuring illumination).

The natural lighting of the premises depends on:

• on the terrain where the building is located. Defines the concept of light climate – so called the nature of the variation of illumination in the open air during the day, month, year. Lighting climate is directly dependent on the geographical latitude and height of the sun.
• on the orientation of the building,
• the distance of the building from the shading objects;
• the location of the light openings and their size.

Location: For the best illumination of the furthest points of the rooms, the upper edge of the light opening should be raised as high as possible above the floor, and the furthest point from the window should be no more than twice the height of the upper edge of the opening above the floor.

Size: In residential and office space requirements for the size of light openings are different: in the residential – 1:8 in relation to the area of the illuminated floor, in the office and administrative – at least 1:10. The size of the light aperture is equal to the area of the opening minus 15% of the area accounted for the window devices.

Based on all these factors, the room has a certain level of illumination, which is characterized by the natural illumination index (NI), which is the ratio of the illumination inside the room (Lx) to the illumination outside (Lx), measured by NI in percent ( %)

Natural illuminance factor for residential and public buildings and industrial premises with side lighting depends on the accuracy of the work performed and varies from 1.5 to 2, and for rooms with rough work KEO = 0.5. At the top and the combined lighting in accordance, this coefficient varies from 2 to 7.

## Artificial lighting

Sources of artificial lighting are any lighting devices (lamps, lamps, LED strips).

When defining the performance characteristics of artificial lighting one should pay attention to:

• light power,
• uniformity of illumination,
• absence of harsh shadows and glare.

## Operating factor

(inversely proportional to the safety factor, KZ, used previously)

When planning illuminance in the design phase, it is important not to forget that during the operation of any luminaire can reduce the illumination it creates. To compensate for this decline, an operating factor (EF) is introduced into the design.

EF for artificial lighting takes into account:

• contamination
• non-recoverable change in reflective and transmissive properties of optical elements
• decrease in luminous flux
• failure of light sources
• contamination of surfaces of rooms, exterior walls of buildings or structures, the roadway or street.

EF for natural lighting takes into account:

• contamination and aging of translucent fillings in light openings,
• reducing the reflective properties of the surfaces of the room. As an example, with dusting of enclosing surfaces in laboratories lighting is reduced by 10% per year, in woodworking shops by 30% for six months.

Measurement of workplace illumination is carried out together with measurements of noise, dust, pollution, vibration – in accordance with SanPin (sanitary rules and norms).

## Measurement of illumination is performed by a Luxometr (from Lux)

Luxmeter is a mobile, portable instrument for measuring illuminance, the principle of operation of which is identical to a photometer.

Rules of use:

• the device is always in a horizontal position;
• it is set in the points, the location of which are calculated according to the method specified in the State Standards. The number of control points must be at least 10;
• all luximeters are certified, and the error of the luxmeter, should not exceed 10%.

Luxometers are subjective and objective.

The subjective luxmeter is based on equating the brightness of two light fields (the illuminance of one field is known). It consists of a gate photocell and a measuring device. The electric current, which the photocell gives when illuminating its surface, is proportional to its illuminance. Therefore the measuring device, which is graduated in lux, shows at once the value of illuminance.

Objective luxmeters are more accurate, they have a selenium photocell as analyzer and a galvanometer to record the reading. When light strikes the receiving part of the photocell, an EMF proportional to the illuminance level appears in the instrument’s circuitry. The scale of the device has 50 divisions, indicating three illuminance measurement limits: 0-25, 0-100, 0-500 lux. If the illuminance exceeds 50 lux, an absorber is installed on the photocell, which extends the basic measurement limits up to 100 times, thus making it possible to measure 0-50 000 lux.

Measurements are taken separately for artificial and natural light. Make sure that no shade falls on the instrument and that there is no source of electromagnetic radiation nearby. This will interfere with the results. After all the necessary measurements of illumination, on the basis of the results, using special formulas, the required parameters are calculated, and a general assessment is made. That is, the parameters obtained are compared to the standard, and a conclusion is made about whether the illumination of a given room or territory is sufficient.

For each type of measurement in each room or area of the street a separate protocol is filled out. Evaluation protocol is issued for each room or area, as well as for the entire site. Illuminance measurement must be done according to the rules.

## Recommendations for measuring lighting conditions for LED luminaires

Measurements of illumination of LED luminaires should be made after their 2-hour operation, when they come up to operating mode (several times during the day). LEDs and power supplies emit a lot of heat. It is removed by heat dissipating materials (aluminum, compound, etc.) and a certain design (large radiator area, etc.). Nevertheless, higher temperature conditions have a serious impact on the illumination.

In order not to make a mistake with the parameters of illumination, it is better to design immediately lay the coefficient of light incidence, which depends on the type and characteristics of the object.

Monitor the performance of LED luminaires and lighting parameters throughout the warranty period, because if the manufacturer declares a warranty period of 3 years or more, the luminaires must retain quality during the entire period if the conditions are met.

If the operating conditions of luminaires occur at temperatures above +45 g, then the measurement of illumination should be done much more often than the norms regulate.