General Question

2davidc8's avatar

Why is a laser scanner not able to read a bar code on my iPhone?

Asked by 2davidc8 (4981 points ) December 29th, 2012

So, I got this discount coupon via email. I printed it out as usual. But since I find it kind of inconvenient having to print coupons, I decided to try an experiment. When I went to pay for my purchase, I asked the clerk to try scanning the coupon on my iPhone. I did not work. But scanning the printed coupon worked just fine. Why is that?

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12 Answers

Lightlyseared's avatar

Laser scanners can’t read the screen as the light reflects off the glass.

LuckyGuy's avatar

The screen is constantly updating and refreshing itself. Like a TV It does it so quickly you cannot see the effect but the laser scanner is fast enough to have a problem with it.
Next time try this experiment. Try reading with the phone in line with the scanner. If that does not work turn one or the other 90 degrees and try again.

dabbler's avatar

There may be polarizing layers on the phone display that might be trapping the laser from reflecting back. It’s worth trying the 90 degree turn of the phone or the scanner that @LuckyGuy recommends to see if that works.

Lightlyseared's avatar

The other possible explanation is that the bars produced by the pixels are not precisely the right size due to the fixed size of the pixels. Imagine one bar needs to be .9mm wide and another is 1.1mm wide but the pixels are 1mm both bars would be displayed incorrectly as 1mm wide (it’s a very crude example but you get the idea).

hearkat's avatar

I use the CardStar App for my rewards cards – I detest carrying extra stuff around, not even a purse if I can help it.

There are specific bar code scanners that can read the phone screen – at Starbucks and CVS, for example. Other stores it seems to be the hand-held scanners that have better success than the ones built in to the checkout counter… which is the case at PetCo and Stop & Shop.

I also have the impression that it has to be an App for this purpose – not just a PDF of the bar code – perhaps they render the codes differently via the apps to make them more readable. I seem to recall one emailed coupon where you could choose either a printable version or a mobile version of the coupon – might have been Bed, Bath & Beyond or Staples – the retailers are also concerned about making sure certain coupons can only be used once, so the mobile option presents that challenge, too.

It seems that increasingly more shops are getting mobile functionality. Check the websites of the stores you frequent to find out whether they have an app or partner with any apps for this purpose.

2davidc8's avatar

Thank you all for your answers. I guess what you are saying is that scanning the phone may work. I’ll try the 90-degree trick.

@Lightlyseared I don’t think the exact size of the bar code matters. I see bar codes of all sizes on various products. Also, I can change the size of the bar code on my printer, and it doesn’t seem to matter. As long as it was printed, it’ll work.

dabbler's avatar

I think @Lightlyseared means the relative proportions of the widths of the lines in the bar code not the overall size of it.

2davidc8's avatar

@dabbler So are you and @Lightlyseared saying that the relative widths of the bars might be different on the phone screen vs. printed?

dabbler's avatar

Yes, because the way the image is scaled on a screen might compromise pixel-width. I.e. the image may be compressed or expanded, losing some pixels off an edge (or adding them) in ways that make it hard for the scanner to interpret what it picks up, because the compression / expansion might not be done uniformly for all lines.

Lightlyseared's avatar

An iPhone 5 has a resolution of 326 pixels per inch where as an inkjet printer can easily have a resolution of 4800ppi (on the basis that that was the resolution of the first hp i googled) and so is much better able to accurately reproduce a barcode at what ever size you want.

hearkat's avatar

I think that if the image of the bar codes is scalable – like a .pdf or .jpg that you can pinch-to-zoom on the iPhone – there may be some distortion of the spacing and thickness of the bars that occurs, similar to when you try to scan a code on a bag that is not perfectly flat.

Apps like the Passbook that is built-in to iOS 6, and Card Star and the Starbucks app have you enter the number or scan the code into the phone, and they produce the bar code on the screen, so it is not a zoomable image. I also have the impression that the screen brightens so that there is maximum contrast between the white of the screen and the black of the bars.

I have used these apps on phones with and without a screen protector on them, so I don’t think the glass is the issue.

2davidc8's avatar

OIC, @dabbler, @Lightlyseared, and @hearkat. Thank you for your input!

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