Page 4 of 4 FirstFirst 1234
Results 31 to 37 of 37

Thread: Using a concave lens to increase divergence - picking the right strength

  1. #31
    Join Date
    Aug 2008
    Location
    UK
    Posts
    5,704

    Default

    Quote Originally Posted by mccarrot View Post
    I just checked, the concave lenses I have are -2,7 This increases the divrigence enough to be at 10MPE for my 500mW at 8 meters distance.

    So for small venues its perfect.

    I also demonstrated the lenses at the last snowLEM. You would think fat beams would destroy the whole laser effect, but it stays pretty nice to watch.

    I think greenalien also got some of these lenses
    Have you tried higher power lenses at smaller distances?

    Myself and Jem had a brief play a while ago with lenses up to -12 diopters on the 500mw I had and the beams looked Ok but I'd have to concede the distance was probably only 2 metres as we were in a small office at the time.

    I was just wondering at what lens power point and distance the beams start to become so fat that the show breaks down.

    A local event to me currently uses a 1W laser (I'm told could be making 1.4-1.5 watts) with no power reduction and the nearest person maybe 20 feet from the beams. Although no one has been hurt and although no camera damage has occurred, I'd like to try increasing safety by offering some advice.

    As the venue is very small maybe only 60 feet end to end (dance floor area alone) I was kind of wondering what the highest power of lens you could get away with in that kind of area without destroying the effect.

    BTW I in no way condone the use of a 1-1.5 Watt laser in this size venue or so near to the crowd. However, I have no connection with the organisation nor any influence over the organisers although I do know them at aquiantance level.

  2. #32
    Join Date
    Apr 2008
    Location
    Amsterdam, NL
    Posts
    2,098

    Default

    Quote Originally Posted by White-Light View Post
    A local event to me currently uses a 1W laser (I'm told could be making 1.4-1.5 watts) with no power reduction and the nearest person maybe 20 feet from the beams. Although no one has been hurt and although no camera damage has occurred, I'd like to try increasing safety by offering some advice.
    You wont get "hurt" or "blind" that fast, but you can almost sure people have had "changes" to the eye. but most people would not even recognize those changes, as a normal person would not know the difference between a floater, and eye damage caused by lasers.

    But I'm not sure if its wise to put this kind of information on a public forum, because maybe people will think a floater is caused by a laser.

    I will Put the -2.7 lens in front of my 700mW Brick and measure the divrigence before and after.
    But I think a bigger value of -3 is unusable unless the location is really small.

  3. #33
    Join Date
    Apr 2008
    Location
    Amsterdam, NL
    Posts
    2,098

    Default

    Here we go,

    Laser used, Viasho 650mW putting out 500mW after the apperture (dirty galvo mirrors)

    Beam size at apperture: 2mm

    Beam size at 5 meters: 5mm

    Beam size at 5 meters with -2,7 concanve aprox 30mm

    mw/cm2 at 5 meters without concave: 1700mw/cm2!! (this is 170xMPE)

    mw/cm2 at 5 meters with -2,7 concave 170mW/cm (17xMPE)

    (measured wih a coherent fieldmax TOII, ultra sensitive PM3 head and a 7mm aperture)

    So you can see the power is reduces by a factor 10 with a -2.7 lens.

    Can someone calculate the divergence? i'm too lazy to find the formula
    Attached Thumbnails Attached Thumbnails Divrigence 2..jpg  

    Divrigence 1..jpg  

    Divrigence 3..jpg  


  4. #34
    Join Date
    Dec 2008
    Location
    Southport, UK
    Posts
    2,746

    Default

    5.599 mrad .
    http://img62.imageshack.us/img62/3985/laser.gif

    Doc's website

    The Health and Safety Act 1971

    Recklessly interfering with Darwin’s natural selection process, thereby extending the life cycle of dim-witted ignorami; thus perpetuating and magnifying the danger to us all, by enabling them to breed and walk amongst us, our children and loved ones.





  5. #35
    Join Date
    Apr 2008
    Location
    Amsterdam, NL
    Posts
    2,098

    Default

    Thanks,

    I also found a calculator:
    http://www.pseudonomen.com/lasers/ca...alculator.html



    So without the lens the laser got 1mRad divergence, and with the lens 5mRad divergence and decreasing the exposure by a factor 10.

    So a -2,7 concave is a good lens to have with your laser gear.

  6. #36
    Join Date
    Aug 2008
    Location
    UK
    Posts
    5,704

    Default

    Very interesting and many thanks for doing the tests, much appreciated.

    Although its probably not a linear response, a rough guesstimation based on a linear response and your figures would tend to suggest that the laser if 1.5w is probably at 510x MPE!!! (3x170x result).

    I guess the only question that remains now is what effect stronger lenses have on both MPE and the visible beam quality at the furthest point at the back of the room. Being such a small venue I'm sure they could get away with a much more powerful lens but I wonder at what point the visible quality of the beam breaks down to a fuzzy rather than sharp beam.

  7. #37
    Join Date
    Apr 2008
    Location
    Amsterdam, NL
    Posts
    2,098

    Default

    More info:

    My 500mW viasho with 500mW after the aperture window did 6MPE (60mW/cm) (yes I have a working scanfail) after 30meters distance.

    The laser got a aprox divergence of about 1,5mW and after 30 meters the beams already started to look fat.

    So a -2,7 lens at this distance will look ugly

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •