Topic 2: Lens Treatments and Lab Processes Flashcards
state the 4 lens treatments
tinting, hard coat, anti reflective coat, top coat
explain multi coat
hard coat, ar coat, top coat on front and back surfaces
lens must first be bare for application, hence multi coated lens cannot be tinted
multi coated lens with tint only through custom made
state the sequence of application
dyes/tint -> primer -> hard coat -> AR -> top coat
state function of tinting lenses
to attenuate (reduce intensity) the light reaching the wearer
explain the 2 ways to add dye to lens
(1) hot bath: (lens dipped into boiling water containing dyes) with the heat, dye particles enter underneath the lens surface. impact resistant lenses have HIGHER RI (pc 1.59, 1.6), LONGER application. LOWER RI (cr39 1.49), SHORTER application
(2) tint coating: coloured film sits on top of lens. manufacturers opt for this during mass production
explain photochromic tint
the tint darkens in the presence of UV and returns to being clear when UV source is removed.
photochromic tint DOES NOT = more UV protection. most plastic lenses already block UV lights naturally.
photochromic tint is so that wearer does not need separate pair of glasses for glare reduction purposes (aka convenience)
state function of hard coat
makes plastic lenses more durable to resist scratches
scratches cause discomfort for vision. MUST HAVE for all plastic lenses
explain function of AR coat
reduces the amount of light reflected on lens surfaces (front and back) for efficient light transmission
reflection on lens surface increases with HIGHER RI, FLATTER surface eg. flat pc 1.59
ALL un cut high index ( > 1.59) plastic lenses pre treated with AR coat
state other benefits to AR coat
removes mirror effect on front lens surface, hence more light transmitted, and facial features visible to observer
reduces headlight and road surface glares, esp important for night driving
blocks 20% of HEV (high energy visible lights which can cause macular degeneration, eye strain and insomnia) - sun, indoor lighting, mobile devices
state the 2 conditions/principles of AR to eliminate reflections completely
path and amplitude condition
explain the 2 principles of AR
(1) path condition: (2 waves exactly out of phase) AR coat thickness = wavelength of light ÷ 4(coating RI)
(2) amplitude condition: (2 waves of reflection equal in strength) RI of AR coat = square root (lens RI)
explain in terms of light waves when AR coat RI is right for the lens material
(1) light hits the lens surface, some reflected & some enter and get reflected at the next lens surface
(2) the 2nd wave of light is delayed timely to cancel 1st wave completely (this is destructive interference where 1st wave peak meets with 2nd wave trough)
explain treatment to add hard coat
primer is prepares lens for hard coat and improves adherence of subsequent coatings to lens
a primer layer increased wettability of lens surface (which is hydrophobic) so that coating materials can be applied evenly and adhere tightly
explain function of top coat
allows surface to be
(1) anti-static: prevent dust sticking
(2) hydrophobic: reduce water marks
(3) oleo-phobic: prevent fingerprints
explain instructions to give wearer on lens multi coat maintenance
(1) keep spectacles in cases when not not in use, to prevent dropping/knocking on hard surfaces
(2) avoid high heat places to prevent crazing (crack lines)
(3) use mild hand soap instead of chemicals to clean lenses
(4) use only micro fiber to clean/shine lenses
