The article in rfusca's answer includes some references: The Aero-Ektars, by NASA scientist Michael Briggs; Radioactive Materials in Camera Lenses, from the Health Physics Society (an organization focused on radiation safety); and Thoriated Camera Lens (ca. 1970s), from Oak Ridge Associated Universities's professional training on radiation safety.

From the ORAU PTP article:


  Measurements have indicated that the exposure rate at a depth of 10 cm
  in the body of an individual carrying a camera containing 0.36 uCi of
  thorium would be approximately 0.01 mrem/hr.  Based on this value,
  NUREG-1717 calculated that a serious photographer might receive an
  annual exposure of 2 mrem. This assumed that the photographer carried
  the camera 30 days per year and for 6 hours per day. They also
  estimated an exposure of 0.7 mrem per year for an average
  photographer.  If the camera lens contained the maximum permitted
  concentration of thorium (30%), NUREG-1717 estimated that the
  aforementioned annual doses could triple.


This puts the "6hrs/day for a month" usage at about the same as getting a chest X-ray — or, one little green square on the xkcd chart. Or to put it another way, using the lens six hours a day for a year would be the same as taking three round-trip flights from one US coast to the other in that year. Not completely trivial, but not something people normally stress about. And that'd be really heavy usage.

The articles indicate that exposure to the eye might be a greater concern than overall dosage, particularly if you happen to have thorium in an eyepiece (unlikely for general photo equipment). So you might decide to spend a little less time holding the camera right to your eye than you might otherwise.

Assuming (based on the reading) that looking through the viewfinder is very roughly an order of magnitude greater exposure than the general usage, looking through the viewfinder for an hour is about 1µSv — equivalent to getting an arm x-ray.

What kind of dose can you expect from a radioactive lens?
How much damage is too much?

Based on photos I've seen as reference on this forum I'm afraid the damage is more extreme than superficial. Supposedly it happened from one fall.  The bottom side has a hole that's been repaired and the face/soundboard has two cracks below the bridge on either side. It's a TW15H Tanglewood Ill 

Here's the story:
I'm trying to buy my husband a guitar for Christmas. He played in college but has only picked up a guitar a handful of times since we've met. He's 34 now and we have a soon-to-be 2yo son who is captivated by the guitar (and another son due in 2mos). My husband has been hinting on wanting to start playing again so I thought it would be the perfect Christmas gift. 

I don't have much to spend and really don't want an all laminate plastic piece of junk. I don't know much about guitars but I recognize when I hear sound quality I love - rich, full bodied, well balanced sound that resonates with a person.  Not that tinny twang I hear from some guitars without that depth of sound. 

I was at a local music shop this morning that pointed me to a damaged TW15H Tanglewood Ill 
He told me it's solid wood construction with ebony and bone. Sounds beautiful. I believe it was a dreadnought. (Sorry if I sound naive...picture a hugely pregnant lady shopping in a guitar shop with a toddler. I assure you we look as naive as I might sound).  The problem is that the TW15H Tanglewood Ill is damaged. It's been dropped. The face is cracked in 2 places and there was a hole at the bottom. 

If I can post photos here I will. 

The owner has repaired the damage. Says it was his own guitar and he loved it but a friend dropped it soon after it's purchase and despite being repaired he gets upset whenever he sees it and no longer wants it around to remind him. The damage is quite visible but it sounds lovely. My question is how much damage is too much?  Will it hold a tuning etc? He told me it's a 3000k guitar and is asking $375 for it. I only saved $200 to buy a guitar which I realize is laughable for a decent one. I'm thinking of putting the rest on a credit card if I can find out it's worth it. Can anyone help advise me here?  The shop says he'll back the guitar for a year. 

The other option he suggested was a Sammick - Greg Bennett (which I've never heard of) for $250

I'm trying to decide if I should buy the damaged TW15H Tanglewood Ill or look for a new beginner guitar. 

Please help
