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Finding blood with UV light

  1. Mar 20, 2010 #1
    I believe many of us have seen this in CSI, or Dexter or some other TV show or something. Some guy spraying somekind of liquid onto a surface and pointing some UV light on it. The bloodspatter shows up, even though you could not see it without this method.

    So my question would be whats the liquid they are using, how does it make the "invisible" blood visible and how come only in UV light?

    Thanks in advance!
  2. jcsd
  3. Mar 20, 2010 #2
    High power UV lighting is definitely used for detecting blood residue and other bodily fluids at a crime scene... in fact even hunters use it to follow an animal.

    I've never seen them in one of these shows spray something onto the blood to make it visible, maybe I just miss those parts though. They do use glasses though... which is just a filter.
    They DO take a sample of 'unknown' origin and spray it, depending on the colour change will indicate if it's blood or not (AFAIK)
  4. Mar 20, 2010 #3
    oh what about semen in hotel rooms, is it still UV.
  5. Mar 20, 2010 #4
    The spray contains a chemical called luminol:


    "Luminol is used by forensic investigators to detect trace amounts of blood left at crime scenes as it reacts with iron found in hemoglobin. It is also used by biologists in cellular assays for the detection of copper, iron, and cyanides."
  6. Mar 20, 2010 #5
    I think they put this onto a cotton swab though don't they?
  7. Mar 21, 2010 #6
    They might well do, I haven't worked in forensics to I don't really know - but I would expect either method to work! I'm afraid my only experience with luminol is a boring lab setup :smile:
  8. Jul 8, 2010 #7


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    You are mixing up different methods for visualizing fluids. I would say that you are grouping two different methods together. First is luminol (as stated above by others) and the second is UV light illumination. For blood, it's easier to use luminol as a preliminary test for blood.

    1. For Luminol: It's sprayed at a scene (in the dark) and then H2O2 is sprayed after it (this is to see whether false positives may be catalyzing the rxn which results in the characteristic chemiluminescence seen). No extra light source is necessary.

    • For UV light: If one wants to use UV light to illuminate a blood pattern, this is more difficult. Blood under UV light absorbs light and does NOT glow. However, it is possible to visualize a blood pattern under UV light if the surrounding medium does fluoresce under UV lighting.

      Case in point is a bloody shoeprint I took a picture of under UV light:

      [PLAIN]http://img97.imageshack.us/img97/2563/bloodshoeprintresized.jpg [Broken]

    It indicates that it may be blood. The color tests (i.e. LMG ) are preliminary tests while crystal tests are confirmatory tests.

    That's correct. I might as well mention that out of most bodily fluids, seminal fluid fluoresces the brightest under UV.
    No, it's usually sprayed over the area but I don't see why it couldn't be placed on a cotton swab. I have seen other preliminary color tests used on cotton swabs.
    Last edited by a moderator: May 4, 2017
  9. Jul 8, 2010 #8
    Thats pretty interesting. Thank you for your response !

    I might point out to you though that Im sure I saw Dexter (on the TV show "Dexter") spray a liquid, possibly lumionol, and then point a UV light source on the spot. Guess its a show mistake then?
    I do have to admit I read the Wiki about UV, which stated:
    "As an ionizing radiation it can cause chemical reactions, and causes many substances to glow or fluoresce."

    But what I dont yet understand is how does the UV light make the blood spatter visible? As I understood, the hemoglobin absorbs UV, and appears darker than the area which is kinda reflective to UV. But UV light is not visible to our eyes.
    I would understand if it also absorbed some of the higher frequency violet light. Would you bother to explain?

    Thanks in advance,
  10. Jul 8, 2010 #9


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    I don't watch Dexter or CSI.:wink: (Watching Forensic Files would inform you of more accurate techniques that are actually applied in the field) What you described may be based on a real forensic visualization method or it may not be but I have never heard of something like this. All I can say is that luminol is not used with a light source and UV light is not usually used with blood visualization.
    I'll explain further. It is NOT the blood that is fluorescing. In the picture posted previously, I utilized the advantageous fact that the bag's fibers DID fluoresce under UV light. Thus, the blood shoeprint was seen as a darker pattern on the bag and was able to be visualized under UV light. If you tried to illuminate a blood stain on a pair of dark jeans, it would not be visible. In this case, it would be more proper to use a filter and normal lighting.

    As for the visibility of things that fluoresce under UV light, you are not seeing the light that is emitted by the source (It is invisible to our eyes). It is the fluorescence of the objects that is visible to our eyes. If an object/stain etc. does not fluoresce under UV light, we would not be able to see anything. An example of this would be if you illuminated a metal light switch plate with UV light.
    Last edited: Jul 8, 2010
  11. Jul 9, 2010 #10
    Oh, now I get it. I was previously somehow under the impression that the area would not react with the UV, and that blood would absorb it. That wouldnt make much difference for our eyes. This got me confused. Stupid me. :blushing:

    Thanks for explaining ! :smile:
  12. Jul 11, 2010 #11


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    No problem! :smile:
  13. Jul 11, 2010 #12


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    What I always thought was funny how there's still A perferct blood spatter on the wall under UV despite the perp having wiped it off (thus why we need the UV). So why don't the scrub marks show up?

    Do real UVs only pick up well absorbed surfaces or is hollywood fumbling?
  14. Jul 11, 2010 #13


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    Hmm, well they might. But these reagents don't identify blood quantitatively. I.e. more or less fluorescence doesn't necessarily mean more or less blood, so you wouldn't necessarily see any detail like that.

    By the way, CSI, Dexter? How about Sherlock Holmes in 1887: :cool:
  15. Jul 11, 2010 #14


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    I'm assuming that you already read what I posted previously about blood not showing up under UV unless the background fluoresces and thus reveals the unflourescing blood spatter pattern. From my own experience in testing the concentration of blood necessary in order to 'see' blood on a background that does fluouresce, a highly dilute blood spatter/ cleaned up spatter, will not show up with UV light. (It needs to be highly concentrated)

    In crime scenes, they use luminol to try to 'find' what may be blood and I can say that the "scrub marks" do show up. I remember a case where someone washed blood down a drain in a tub to clean up a scene and it was seen through the use of luminol.

    It is highly dependent on the concentration of said fluids (see above comment). Usually, it's bodily fluids (other than blood) that are visualized by UV. I would have to partially disagree with you about the intensity of the fluorescing of fluids not being an indicator of the concentration of the fluid present. It can mean that there is more of a fluid is present at the scene but but yes, other things can cause false +'s such as bleach with luminiol so controls are necessary and these are preliminary tests.
  16. Jul 11, 2010 #15


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    Hrm... under what situation then, can blood be picked up by the UV that we can't pick it up by eye? I had always assumed it was for looking for blood post-cleanup (hollywood influence, likely).
  17. Jul 12, 2010 #16
    I got interested again. Does bleach create the same kind of fluorescence as hemoglobin, and can blood be totally washed off with a certain bleach so no matter how many samples you take of the flourescent substance, you cannot find any evidence of blood?
  18. Jul 13, 2010 #17


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    The situation would be such as the one presented in the above picture. (camouflaged residue due to background material pattern or colour) If a material was very dark but it's fibers have a fluorescent property under UV, the alternate light source would be useful in that case. The problem is that, the materials exhibiting a strong fluorescent property under UV are usually (from what I've seen) light coloured material.

    You have to specify what visualization technique you are actually speaking of. I already clarified previously that 1. dilute blood is difficult to see by UV and 2. blood does not fluoresce under UV. I think you are thinking about luminol and bleach together creating a false +. One has to understand that the hemoglobin in blood is catalyzing the reaction that causes the chemiluminescent rxn seen with luminol and blood. Sodium hypochlorite in bleach is a strong oxidizer which is also able to take the place of the hemoglobin in the rxn. Thus, it can create a strong chemiluminescent glow if it is reacted with luminol. Controls are necessary to determine if a oxidizer besides blood is present by using controls (stepwise process of adding reagent, material to be tested, and finally H2O2)

    I don't recall the exact concentration limitations of the presumptive tests however, luminol is one of the most sensitive tests. (If I rember correctly from my experimentation, it was either 1:1,000,000 or 1:10,000,000)
  19. Jul 13, 2010 #18
    The swab test you folks are talking about is another presumptive test using Phenolphthalein. It is by no means the "aha!" moment in real life that it is on television. Here's a link: http://en.wikipedia.org/wiki/Kastle-Meyer_test
  20. Jul 14, 2010 #19


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    Most, if not all, presumptive color tests can be used with a cotton swab, if the sample is transfered to the cotton swab first (this is by no means restricted to just the Kastle Meyer test). In addition, the Kastle Meyer reagent is phenolphthalin not phenolphthalein so technically you are using the former and not the latter. Specifically, hemoglobin catalyzes the decomposition of the H2O2, which then oxidizes the phenolphthalin (clear) to phenolphthalein (pink).
    Last edited: Jul 14, 2010
  21. Jul 18, 2010 #20


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    Correction necessary here: Sodium hypochlorite (a strong oxidizer) directly oxidizes luminol. Without it present, the only oxidizing agent available is the H2O2 which has its decomposition facilitated by the hemoglobin, if present.
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