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Thermal Relaxation Time - TRT

Time taken for 50% of heat energy to be conducted away from target tissue. Optimum pulse duration should be more than TRT of epidermis & shorter than TRT of hair follicle conferring heat to the structure. TRT is related to the square of diameter of target tissue.

  • For skin –TRT is 3-5 ms.
  • For hair follicle TRT is 10-50 ms.

Cooling helps to dissipate heat away from the epidermis leaving deeper follicles more vulnerable.

  • Ideal Pulse width 10-65ms.
  • SPOT SIZE – width of beam 

Penetration depends on the spot size.

  •  Small spot size – less penetration
  •  Large spot size – more penetration


Energy delivered per unit area measured in J/sq.cm.  Fluence tolerance is inversely proportional to skin colour. Darker the colour fluence is decreased to reduce heating of epidermis, Darker hair requires less fluence. Lighter hair has less melanin & hence requires more fluence.


Fluence can be increased by increased energy output & small spot size.



Practical Aspects

  • Underlying cause for excess hair should be corrected , like hormonal imbalance, medical conditions leading to hair growth..
  • No plucking, waxing, threading, bleaching 15 days before each session. Shaving and hair removing creams could be used instead.
  • Using ice packs, topical anaesthetic creams or decreasing energy minimizes discomfort during treatment.
  • Mild discomfort like redness, burning sensation is taken care with topical emollients and ice packs.
  • Sun exposure to be avoided and use of a sunscreen (minimum SPF of 15) is advocated.
  • Post later skin is not as after waxing or threading. The hair follicle destroyed by the laser are lifeless and shed off over a period of time. Only growing follicles are destroyed by laser and rest of the hair follicles continue to grow.
  • Check the growth of the hair & roots before and after trimming. Look for the slight erythema, oedema and charred roots of the hair follicles after the procedure.


IPL Crystal Filters:

480 nm: Acne.

530 nm: Skin rejuvenation, photofacial, fine wrinkles.

590 nm: Hyperpigmentation, increased vascularity, melasma, sun spots, lentigenes.

640 and 690 nm: Hair reduction, hypertrichosis.




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