illumination part-2

Topic includes

- Sodium vapour lamp

-  It's working

- It's usage

-  Mercury vapour lamp

- It's usage

- Street lightening

- Principle and Objective

- It's types 

Illumination part 2

Sodium vapour lamp

A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589 nm.

Two varieties of such lamps exist: low pressure and high pressure. Low-pressure sodium lamps are highly efficient electrical light sources, but their yellow light restricts applications to outdoor lighting, such as street lamps, where they are widely used. High-pressure sodium lamps emit a broader spectrum of light than the low-pressure lamps, but they still have poorer color rendering than other types of lamps.Low-pressure sodium lamps only give monochromatic yellow light and so inhibit color vision at night.

How does a sodium Vapour lamp work?

Sodium vapour lights are a type of gas-discharge lamp. They generate light by sending an electrical charge through a gas to produce a plasma. ... The wavelengths (colour) of this light are characteristic of the atomic element in question. Low pressure sodium gas produces mostly light in the yellow-red part of the spectrum.

Where are sodium Vapour lamps commonly used?

High-pressure sodium lamps (also known as HPS Lamps or HPS lights) are a type of sodium lamp that is widely used in industrial lighting and many public outdoor areas. They are commonly used in public parking lots, roadways, and other security areas.

Mercury vapour lamp

A mercury-vapor lamp is a gas discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger borosilicate glass bulb. The outer bulb may be clear or coated with a phosphor; in either case, the outer bulb provides thermal insulation, protection from the ultraviolet radiation the light produces, and a convenient mounting for the fused quartz arc tube.

Mercury vapor lamps are more energy efficient than incandescent and most fluorescent lights, with luminous efficacies of 35 to 65 lumens/watt.

What are mercury vapor lamps used for?

A mercury vapor lamp is a type of high intensity discharge (HID) lamp that is typically used for commercial and outdoor lighting. It was the original metal vapor style of light fixture to be mass produced for general purpose lighting.

How many types of mercury Vapour lamps are there?

There are five lamps with phosphor coating to provide improved color performance. As the wattage increases the initial lumen ratings for the phosphor coated lamps get available with 4200, 8600, 12100, 22500 and 63000 ratings. The average life of mercury lamp is 24000 hours i.e. 2 years 8 months.

Objective of Street Lighting:

The main objective of street lighting are:

(i) To make the traffic and obstructions on the road clearly visible in order to promote safety and convenience.

(ii) To make the street more attractive.

(iii) To increase the community value of the street.

The principle employed for street lighting is different from that of interior lighting. There are no walls and ceiling which reflect or diffuse light, hence only direct lighting scheme can be employed and hard shadows and high contrast cannot be avoided. Because of areas to be illuminated being large, the value of illumination for economic reasons, is very low compared to that for indoor lighting and the question of colour rendering is also of minor importance.

High level of illumination is not necessary, because firstly one has seldom to look continuously at nearby objects, secondly it is the impression of whole scene that is required. In fact in case of interior lighting the objects are seen by the light reflected by them but in case of street lighting the objects are seen in relatively bright back ground i.e., silhouetted against a relatively bright back ground. Furthermore, owing to the low illumination, the eye is in its most sensitive state and, therefore, glare must be avoided.

Principles of Street Lighting:

Two general principles are usually employed in the design of street lighting installations, namely:

(i) The diffusion principle and

(ii) The specular reflection principle.

1. Diffusion Principle:

In this case the lamps fitted with suitable reflectors are used. The reflectors are so designed that they may direct the light downwards and spread as uniformly as possible over the road surface. In order to avoid glare the reflectors are made to have a cut-off of between 30° to 45° so that the filament is not visible except from underneath it.

The diffusing nature of the road surface causes the reflection of a certain proportion of the incident light in the direction of the observer, and therefore, the road surface appears bright to the observer. The illumination at any point on the road surface is calculated by applying point to point or inverse-square law method. Over certain proportions of the road the surface is illuminated from two lamps and the resultant illumination is the sum of the illuminations due to each lamp.

2. Specular Reflection Principle:

In this case the reflectors are curved upwards so that the light is thrown on the road at a very large angle of incidence. It is observed that a motorist requires to see objects about 30 metres away. Thus the observer is shown about 30 metres from the object. Much of the light from the lamp L3 is not reflected towards the observer, whereas most of the light from lamps L1 and L2 is reflected towards him. Thus the object will appear silhouetted against the bright road surface due to lamps at a longer distance.

Types of Lamps for Street Lighting:

Mercury vapour and sodium discharge lamps have been found to have certain particular advantages for street lighting purposes; the most important of these is the lower power consumption for a given amount of light, which, inspite of the higher cost of the lamps makes the overall cost of an installation with discharge lamps less than that employing filament lamps. The colour and monochromatic nature of the light produced by discharge lamps do not matter much in street lighting installations.