System Components for Industrial Illumination

Stroboscopic effect is the illusion in which a rotating or moving object appears to be stationary, moving slowly, or moving backwards when lit by a light source whose output flickers at a regular rate. Discharge lamps (fluorescent, sodium, mercury) and poor LED drivers on 50 Hz supply give light that rises and falls 100 times per second. If a machine part rotates at a speed that is a multiple of this flicker, it looks still, which is dangerous in factories because workers may touch a running lathe chuck, fan or saw.

Prevention in factory lighting

1. Distribute lamps over three phases: adjacent luminaires (or the tubes of a twin/three-lamp fitting) are connected to R, Y and B phases. The light dips of each phase occur at different instants, so the total light is nearly steady.
2. Lead–lag (twin-tube) circuit: in a twin fluorescent fitting, one tube is fed through an inductive ballast and the other through a ballast with a series capacitor, so their currents (and light pulses) are about 60° apart.
3. High-frequency electronic ballasts: operate lamps at 20–40 kHz, so the flicker is far too fast to cause the effect.
4. Flicker-free LED drivers: use constant-current drivers with low ripple (flicker percentage low) instead of cheap capacitor drivers.
5. Local lighting with incandescent/halogen lamps on machines with rotating parts; the filament's thermal inertia smooths out the variation.
6. Mix of light sources in the same area so that the flicker of one is filled by another.

Colour rendering is the ability of a light source to show the true colours of objects compared with a reference source (daylight or an incandescent lamp). It is measured by the Colour Rendering Index (CRI or Ra) on a scale of 0–100; 100 means colours look exactly as under the reference source.

Why colour rendering is important

• Correct identification: colour-coded wires, pipes, safety signs, and medicines must be recognised correctly; poor CRI can lead to errors and accidents.
• Quality control: printing, textile, paint, food grading and hospital work need accurate judgement of colour.
• Visual comfort and appearance: skin tones, goods in shops and interiors look natural and pleasant under high-CRI light.
• Productivity: better colour contrast reduces eye strain and mistakes.

How CRI helps in selecting luminaires

So the designer first fixes the minimum CRI the task needs (e.g. Ra ≥ 80 for offices as per EN 12464 / IS 3646, Ra ≥ 90 for colour inspection), then chooses among the lamps meeting it the one with the highest efficacy and suitable colour temperature.

High Intensity Discharge (HID) lamps produce light by an electric arc through a gas/metal vapour at high pressure inside a small arc tube. They have high luminous efficacy, long life and high light output from a compact source, so they are used for street lighting, factories with high ceilings, stadiums and flood lighting. All need a ballast (and often an ignitor) and take a few minutes to reach full output.

Types: high-pressure mercury vapour (HPMV), high-pressure sodium vapour (HPSV) and metal halide (MH) lamps.

Effect of voltage variation

• HPMV lamp: light output changes about 3 % for each 1 % change in voltage. A dip of more than about 10–15 % may extinguish the arc, and re-striking needs 4–7 minutes of cooling.
• HPSV lamp: lamp wattage and light change roughly 2–2.5 % per 1 % voltage change; overvoltage shortens life, and the arc may go out on large dips, with a 1–2 minute restrike time.
• Metal halide lamp: output varies like HPMV, and the colour also shifts with voltage; low voltage can extinguish the lamp and restrike takes 10–15 minutes.
• Common effects: overvoltage raises arc tube temperature and shortens life; undervoltage reduces output and may cause failure to start. Electronic or constant-wattage ballasts reduce these effects.

For comparison, an incandescent lamp's light varies about 3.4 % per 1 % voltage change and LED lamps with constant-current drivers are almost unaffected.

Types of light sources

1. Incandescent (filament) lamps: a tungsten filament heated to about 2500–2800 °C glows. Includes GLS lamps and tungsten-halogen lamps (halogen gas allows higher filament temperature and longer life). Efficacy 10–25 lm/W, excellent colour rendering (Ra ≈ 100), cheap but short life (1000–2000 h).
2. Low-pressure discharge lamps:
   • Fluorescent tube (FTL) and compact fluorescent lamp (CFL): mercury vapour discharge produces UV, which phosphor coating converts to visible light. 50–90 lm/W, life 8000–15000 h, need ballast.
   • Low-pressure sodium (LPS): monochromatic yellow light, very high efficacy (up to 180 lm/W) but very poor colour rendering.
3. High-intensity discharge (HID) lamps: high-pressure mercury vapour (35–60 lm/W), high-pressure sodium (80–140 lm/W, golden-yellow), metal halide (70–110 lm/W, white light, good CRI). Used for streets, high-bay factories and floodlights.
4. Solid-state lighting – LED: semiconductor p-n junction emits light when forward biased. Efficacy 100–200 lm/W, life 25,000–50,000 h, instant start, dimmable, no mercury. Now the most used source for all applications.
5. Others: induction lamps (electrodeless, very long life), neon lamps for signs.

Lighting schemes for indoor lighting

Based on how much of the luminaire's light goes directly downward to the working plane:

By coverage, indoor lighting is also arranged as general lighting (uniform over the whole area), localised lighting (extra light on specific work points), and general plus local lighting, with emergency lighting for safety.

Supply voltage often varies ±10 % from rated value. The change in voltage changes the lamp power, light output, efficacy (lm/W) and life, and the size of the effect depends on how the lamp is controlled.

a) Incandescent filament lamps

The filament resistance rises with temperature, so the effect is strong. Approximate textbook relations (V = voltage ratio):

Light output  ∝ V^3.4
Power         ∝ V^1.6
Efficacy      ∝ V^1.9
Life          ∝ V^-13

• A 5 % overvoltage raises light by about 18 % and efficacy by about 10 %, but cuts life to about half.
• A 5 % undervoltage lowers light by about 16 % and efficacy by about 9 %, though life almost doubles.
• So efficiency rises with voltage at the cost of life.

b) Compact fluorescent lamps (CFL)

• CFLs have a built-in electronic ballast that partly regulates lamp current.
• Light output changes roughly 1–1.5 % per 1 % voltage change; efficacy stays almost constant over about ±10 %.
• At low voltage (below about 80–85 % of rated) the lamp may not start or flickers; overvoltage stresses the ballast capacitors and shortens life.

c) Fluorescent lamps (with magnetic ballast)

• Light output and power change by about 1–2 % per 1 % change in voltage; efficacy changes only slightly.
• Undervoltage: hard starting, flicker, blackening of tube ends (cathode damage) and reduced life. Overvoltage: ballast overheats, cathodes wear fast.
• With an electronic ballast, output is nearly constant and efficacy is higher.

d) LED lamps

• LEDs are driven by a constant-current driver, so the LED current, light output and efficacy remain almost constant over a wide range (e.g. 100–270 V for universal drivers).
• Cheap capacitor-type drivers do not regulate: light changes nearly in proportion to voltage and flicker increases.
• Overvoltage and surges damage the driver, which is the usual cause of failure; heat also reduces LED life.

Summary

Types of light sources

1. Incandescent (filament) lamps: a tungsten filament heated to about 2500–2800 °C glows. Includes GLS lamps and tungsten-halogen lamps (halogen gas allows higher filament temperature and longer life). Efficacy 10–25 lm/W, excellent colour rendering (Ra ≈ 100), cheap but short life (1000–2000 h).
2. Low-pressure discharge lamps:
   • Fluorescent tube (FTL) and compact fluorescent lamp (CFL): mercury vapour discharge produces UV, which phosphor coating converts to visible light. 50–90 lm/W, life 8000–15000 h, need ballast.
   • Low-pressure sodium (LPS): monochromatic yellow light, very high efficacy (up to 180 lm/W) but very poor colour rendering.
3. High-intensity discharge (HID) lamps: high-pressure mercury vapour (35–60 lm/W), high-pressure sodium (80–140 lm/W, golden-yellow), metal halide (70–110 lm/W, white light, good CRI). Used for streets, high-bay factories and floodlights.
4. Solid-state lighting – LED: semiconductor p-n junction emits light when forward biased. Efficacy 100–200 lm/W, life 25,000–50,000 h, instant start, dimmable, no mercury. Now the most used source for all applications.
5. Others: induction lamps (electrodeless, very long life), neon lamps for signs.

Specular reflection principle of street lighting

When a road surface is wet or smooth, it behaves partly like a mirror (specular reflection): the angle of reflection equals the angle of incidence. A motorist then does not see the road as uniformly lit; instead, each lamp produces a bright elongated patch (a "T"-shaped streak) on the road between the lamp and the driver, and the road between patches looks dark.

The specular reflection principle uses this effect: lamps are arranged so that the bright reflected patches of successive lamps overlap and form a continuous bright background, against which objects (pedestrians, vehicles) are seen in dark silhouette.

 lamp          lamp          lamp
  O             O             O
  |\            |\            |\
  | \  bright   | \  bright   | \
==|==\=patch====|==\=patch====|==\=== road
      \             \             \
       \______________\_____________→ driver's eye

Design rules:

• Use a fairly small spacing and low mounting height so the bright patches join; the patch is long in the direction of travel.
• Luminaires are mounted over the road (on brackets/outreach) and aimed along the road so that maximum light reaches the driver's direction of view.
• Use cut-off or semi-cut-off luminaires to limit glare to the driver.
• The principle works for smooth/wet roads; on rough dry roads diffuse reflection dominates and the diffusion (lumen) principle is used instead.