Incandescent lamps are based on technology that was developed centuries ago. The lamps produce light by passing current through tungsten wire. The emission of rays are primarily in the infrared region as the working temperature of tungsten filaments is about 2700 K. These lamps are inexpensive, easy to use, and free of toxin components. The colour rendering properties of incandescent lamps are also good. This technology has been phased out or is under phase out program in majority of nations.

Key advantages:

  • Cheap and easy to install
  • Good colour rendering
  • Controls can be easily installed

Key disadvantages:

  • High heat loads
  • Low efficacy ( 5 to 15 lm/W)
  • Short lamp life (1000 h)
  • Supply voltage fluctuation could damage the equipment completely
  • High operational cost

The life of an incandescent lamp is short as the tungsten filament gradually evaporates with heat. Tungsten halogen lamp has halogen gas inside the bulb which restricts the evaporation of filament, and evaporated tungsten gets redeposited to the filament through the process of halogen cycle. The process increases the operating temperature but results in better colour rendering index.

Key advantages:

  • Size is small and easy to install
  • Light source has directional light
  • Controls can be easily installed
  • Excellent colour rendering
  • Better lamp life compared to incandescent (2000 – 4000 hrs)
  • Instant switching and shorter time required for switching it on

Key disadvantages:

  • Low efficacy
  • High operating cost
  • Higher heat loads
  • Supply voltage fluctuation could damage the equipment completely

Fluorescent tube is a low-pressure gas discharge light based technology, which is in a form of long tube with an electrode at each end. In fluorescent tube, Ultra-violet radiations are produced by discharge in mercury vapour in low pressure. Further, ultra-violet light is used to activate fluorescent powder to produce light. The major emission of rays are in ultraviolet region because of the working technology. A layer of phosphor inside the tube is used to convert UV radiations into light resulting in a wide range of visible light wavelength. Correlated colour temperature (CCT) varies from 2700 K to 17000 K and colour rendering indices (CRI) from 50 to 95 are available.

Technology of fluorescent tube has developed significantly in last 2 decades providing higher performances with better efficacies. Three categories of fluorescent tube are available in India – old and almost obsolete T12, T8, and most efficient in the category – T5. With each subsequent development, the diameter of the tube has been reduced considerably, the efficacy has increased from 45- 55 lm/W (T12) to 70 – 95 lm/ W (T5), and mercury content has been reduced, and better light output ratio resulting from reduced diameter. The performance of the tubes is sensitive to ambient temperature. It has been noted that the T5 performs best at the ambient temperature of 35 degree Celsius and a T8 tube at 28 degree Celsius.

Key functional uses:

  • Mostly used for general lighting in most working places like offices, hospitals, etc.
  • A special subcategory of fluorescent lamps is called as five- phosphor lamps. This has an excellent colour rendering properties useful for art gallery, shops, museums, etc.

Key advantages:

  • Inexpensive
  • Better efficacies than incandescent lamps. T5 has one of the best efficacies in the market
  • Long lamp life. The life period of T5 is 8,000 – 10,000 hours
  • Large variety of CCT and CRI

Key disadvantages:

  • Ambient temperature affects the performance of the light
  • Requirement of a ballast
  • Contain toxic material like mercury

Compact fluorescent lamps are from fluorescent tube family with a compact size. The tubular discharge tube is also reduced to two to size U bend or spiral shape structure with diameter much smaller than a T5 itself. CFLs have internal ballast and screws. Pin based ballasts are also available which may require an external ballast. The luminous efficacy of CFL is far more efficient than an incandescent and T12.  CFLs are available in many shapes and sizes to fit every type of requirements.

Key advantages of CFLs:

  • Good luminous efficacy
  • Lower heat loads
  • Different CCT
  • Controls can be easily installed

Key disadvantages:

  • Shorter life compared to LED and T5
  • Contain toxic material like mercury

HID technology uses gas discharge to generate optical radiations in form of single spectral lines. These spectral lines may be used directly or via conversion by phosphors. Depending on the spectral lines, different light colours are generated via these discharge lamps. A magnetic or electric ballast is critical to complete the circuit of the discharge lamps and ensure stable operations. The process of light generations produces significant amount of heat which results into thermal loading on the discharge tube wall raising the temperature to nearly 1000 degree Celsius. 

Mercury Vapour
Light is produced in a mercury lamp with electric current through mercury vapour. A discharge in mercury produces light in 5 visible wavelength and other wavelengths are converted to visible by phosphor layers at the outer bulb. The efficacies of these lamps are a range of 40 – 60 lm/W, CRI between 40 – 60 and CCT at nearly 4000 K.Mercury lamps have been banned any few countries due their toxic content and heat intensive operations

Metal Halide
Mercury vapour lamp emits spectral lines in five wavelength only resulting in limited colours. To increase the CRI of mercury high pressure lamp together with efficacy, mixtures of metal components are added to the discharge tube to emit respective line spectra. This process results in variety of light colours. Metal halides (compounds with iodine and bromine) are more suitable and stable compound for the process. The luminous efficacy, CCT, and CRI of metal halide ranges from 50 – 100 lm/ W, 3000 – 6000 K, 70 – 90 respectively. The average lamp life ranges from 4000 – 8000 hours.

Key advantages:

  • Better luminous efficacy compared to mercury vapour
  • Better CRI compared to mercury vapour

Key disadvantages:

  • Relatively expensive
  • Starting and re-staring time is 2 – 4 minutes

Sodium Lamps
Sodium lamp uses sodium vapour to produce golden-yellowish light emission. CRI value of these lamps are very low (nearly 20) limiting the application of the product but the high luminous efficacy made it suitable for street and area lighting for decades. The luminous efficacy, CCT, and lamp life of sodium lamps ranges from 80 – 100 lm/ W, 1800 – 2200 K, 10,000 – 12,000 hours respectively. Methods to improve the CRI are available in the market at the cost of lower efficacy.

Key advantages:

  • High luminous efficacy
  • Long lamp life

Key disadvantages:

  • Low CCT and CRI
  • Starting and re-starting time is 2 – 5 minutes

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