Gas station without pumps

2016 January 18

Reading glasses

Filed under: Uncategorized — gasstationwithoutpumps @ 18:12
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At my last eye examination, my ophthalmologist told me that my astigmatism had gone away—I now needed a simple –4 circular correction in my right eye (my left eye is difficult to measure correction for, as I have essentially no central vision in that eye).

I have almost no accommodation left in my eyes—my focus range without glasses is about 22cm–26.5cm (3.75–4.5 diopters—a diopter is the reciprocal of the focal distance in meters).  With –4 diopter glasses, my focus range should be 2m—∞, which seems about right for my distance glasses (I don’t remember their prescription).  I just bought a new set of computer glasses with –2 diopters, giving me a focus of 36–52cm, which would be 1.9–2.8 diopters after correction, or 3.9–4.8 diopters before correction, slightly further away than I measured directly).

For bicycling, my distance glasses are fine; for screen work, my computer glasses are fine; and for reading paperbacks I can go without glasses. For really close work like soldering, I want reading glasses—even weak ones would let me get close to the soldering iron without having an exposed eyeball that could easily be damaged. Reading glasses would also be a good substitute for a jeweler’s loupe when looking at the markings on tiny capacitors or integrated circuits

So today I went to the discount store about 1/4 mile from my house and bought 3 pairs of reading glasses at $2 each: +1.5, +2.5, and +3.5.

Three pairs of reading glasses at $2 each.

Three pairs of reading glasses at $2 each.

I tried measuring my focus distance (approximately) for each of the glasses:

glasses min focus max focus diopter range imputed eye range (diopters)
 distance (–4?)  1.5m?  ∞
 computer (–2)  35.5cm  51.5cm 1.94—2.82 3.94–4.82
 none (+0)  21.5cm  26cm 3.85–4.65 3.85–4.65
 +1.5  16cm  18.5cm 5.41–6.25 3.91–4.75
 +2.5  13.5cm  16.5cm 6.06–7.41 3.56–3.91
 +3.5  13cm  14.5cm 6.90—7.69 3.40–4.19

None of the focus measurements are very exact—I was using a ruler held against my face for all but the distance measurement, which was guesstimated—but they are consistent enough to be a useful guideline for me.  I expect that I’ll use the +1.5 reading glasses for soldering work for now, and the others I’ll keep in my toolbox for closer work

Note that even with changing glasses, I have a lot of distances that I can’t focus at—and I expect those gaps to get wider as I age and my eyes become more single-focus.

2015 December 18

Examine pictures carefully

Filed under: Uncategorized — gasstationwithoutpumps @ 07:41
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I was considering buying a cheap color sensor board from Elecrow, because it claimed “This module is based on the color sensor TCS3414CS with digital output I2C.”  That chip is a pretty nice one with a good spectral response.

But I took a closer look at the image on the web page:

Elecrow's image of their color sensor board

Elecrow’s image of their color sensor board

Zooming in on the color sensor itself. Note that this is a leadless chip carrier package, not a ball-grid array package.

Zooming in on the color sensor itself. Note that this is a leadless chip carrier package, not a ball-grid array package.

The package that the chip is in is clearly the FN “dual flat no-lead” package and not the CS “6-lead chipscale” package, which is smaller and has no contacts on the sides. Normally, a change of packaging like that makes little difference (other than to manufacturing cost), but the TCS3414CS and TCS3414FN chips have different spectral responses:

The CS package has an infrared-blocking filter that is missing in the FN package, making the FN chip much less useful as a color sensor. (click image to embiggen)

The CS package has an infrared-blocking filter that is missing in the FN package, making the FN chip much less useful as a color sensor. (click image to embiggen) Spectra from the TAOS data sheet provided by Elecrow.

Since infrared light can throw the readings from the TCS3414FN chip way off, I can’t see any point to get a color sensor using the FN package (one would have to add external optical filters, which eliminates any price advantage the FN chip may have).  I’m a little surprised at Elecrow claiming to have a TCS3414CS chip, when their picture clearly shows the less useful TCS3414FN chip.  It makes me suspicious of other claims Elecrow makes.

Note: I’ve had Elecrow make boards I designed and assemble them, and they did fine work—but anything that they designed I’ll want to look at twice to make sure they’re not claiming more than they’re delivering.

2013 May 28

Snell’s Law lab

Filed under: home school — gasstationwithoutpumps @ 23:23
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We are way behind on physics—my son took the AP C: E&M test before we even got to Faraday’s Law.  He read through the last 3 chapters of Matter and Interactions in 3 days, rather than the 8 weeks we had originally planned, and he hasn’t done any of the exercises in those chapters yet.  Because he is planning to take the SAT 2 Physics test this Saturday, I decided that he should at least have a cursory familiarity with Snell’s Law.  Since there wasn’t time or energy for a problem set, we did a lab instead.

I had noticed when playing around with the violet (405nm) laser pointer, that the water in the fish tank fluoresced brightly.

Laser beam causing fluorescence of water in fish tank.  The blue is probably from bacterial cells in the water, and the red from chlorophyll from algae growing on the walls of the tank.

Laser beam causing fluorescence of water in fish tank. The beam comes in from the right and is reflected off the water-air interface.
The blue is probably from bacterial cells in the water, and the red from chlorophyll from algae growing on the walls of the tank.  We get the blue fluorescence even from water right out of the tap, though not as brightly.

We made some very crude measurements of the angle of the beam coming into the water and of the beam in the water using a protractor.  (The beam coming in was invisible in the air, so measuring the incoming angle was very inaccurate.)

Despite the very inaccurate measurements, my son got a decent estimate of the index of refraction of the water.  We don't know the true index of refraction, since the water has a lot of "stuff' in it.

Despite the very inaccurate measurements, my son got a decent estimate of the index of refraction of the water. We don’t know the true index of refraction, since the water has a lot of “stuff’ in it.

Here is the gnuplot script he used for fitting the data (with some editing by me, which he did not entirely approve of):

set angle degrees

set xrange [0:90]
set yrange [0:60]

set title 'Index of Refraction in a Fishtank'
set key top left
set xlabel 'normal angle in (degrees)'
set ylabel 'normal angle out (degrees)'

refract(a_in, rfr_ind) = asin(sin(a_in)/rfr_ind)

water_rfr = 1.333
fishtank_rfr = 1 # initial guess

fit refract(x, fishtank_rfr) 'snell1.gnudat' using (90-$1):(90-$2) via fishtank_rfr

plot 'snell1.gnudat' using (90-$1):(90-$2) title 'Measured', \
    refract(x, fishtank_rfr) title sprintf('Fitted %f', fishtank_rfr), \
    refract(x, water_rfr) title sprintf('Pure Water %.3f', water_rfr)

I’m sure that with more careful measurement, we could get much less scatter around the theoretical curve, but we were tired at the end of the day and couldn’t be bothered to do the measurements right.

2010 September 10

New glasses

Filed under: Uncategorized — gasstationwithoutpumps @ 13:17
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Like many people my age, I have presbyopia, which reduces the range over which I can focus. Because I started out with myopia (near-sightedness), I’ve been able to manage quite well for the past ≈45 years with glasses just for distant vision.  When I wanted to see something close up (like for reading), I took my glasses off.

When I was young, I could focus out as far as about 33 cm without my glasses, which meant that I needed -3 diopters of correction in my lenses to focus out to infinity.  There is a fairly easy computation for the basic correction needed for myopic people:  measure the distance to the farthest comfortable focus d in meters.  The correction needed to allow you to focus to infinity is -1/d diopters.  (Note: this is only a rough guide, since it does not include cylindrical correction for astigmatism.) Those with hyperopia (far-sightedness) don’t have such a convenient measurement, since they have only a closest focus point to work with.

I used to be able to focus in to about 12cm, which would be about 8 diopters.  Since the diopter scale is roughly additive, that meant the near focus with -3 diopter lenses was about 5 diopters, or a near focus of 20 cm with my glasses.  So without my glasses I could do 12–33 cm and with my glasses I could do 20–∞ cm, with a comfortable overlap where I could work either way.

As I aged, though, I lost accommodation (the range over which one can focus) and I now focus from about 20 cm to about 28 cm without my glasses.  Thus, my distance glasses need a correction of about -3.5 diopters.  The near focus corresponds to about 5 diopters, so my near focus with the classes is about 1.5 diopters, or about 67 cm.  The gap from 28 cm without glasses to 67 cm with glasses includes my normal viewing range for computer screen (about 60 cm for my laptop), so I’ve been forced either to sit way back and make my fonts big, or take my glasses off and hunch over to put my eyes close to the screen.

I tried some progressive lenses that were supposed to give me distance correction at the top of my visual field and less correction at the bottom of my visual field, but after trying them for over a month last year, I gave up on them.  Whenever I turned my head, the top half of the visual field moved at a different rate than the bottom half, making me feel slightly queasy.  Also, only a small stripe across the middle of my visual field had the right focus for reading computer monitors, which meant I had to hold my head very still at a precise (and uncomfortable) angle.

This year, I gave up on the idea of having only one pair of glasses.  I now have one pair for distance vision, and a new pair that I just picked up today specifically for computer usage.  The new pair has about 2 diopter less correction then my distance-vision glasses, allowing me to focus from about 25cm to about 50cm.  This almost, but does not quite, fill the gap in my visual range.  It will take me a while to adjust my sitting position to use the new glasses effectively.  I think that if I were to do it again, I’d go for only 1.5 diopters less correction than my distance vision, as I find myself now right at the outer focus limit with the computer glasses and my laptop.

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