<s>
Samples	B-Device
per	I-Device
inch	I-Device
(	O
SPI	O
)	O
is	O
a	O
measurement	O
of	O
the	O
resolution	O
of	O
an	O
image	B-Application
scanner	I-Application
,	O
in	O
particular	O
the	O
number	O
of	O
individual	O
samples	B-Algorithm
that	O
are	O
taken	O
in	O
the	O
space	O
of	O
one	O
linear	O
inch	O
.	O
</s>
<s>
Many	O
scanners	O
use	O
interpolation	B-Algorithm
techniques	O
to	O
achieve	O
a	O
higher	O
effective	O
SPI	O
rating	O
,	O
with	O
some	O
manufacturers	O
offering	O
nearly	O
one	O
million	O
SPI	O
,	O
though	O
the	O
quality	O
is	O
primarily	O
limited	O
by	O
the	O
optical	O
resolution	O
;	O
interpolated	O
SPI	O
does	O
not	O
provide	O
additional	O
pixels	O
in	O
the	O
scanned	B-Application
image	I-Application
.	O
</s>
<s>
Horizontal	O
and	O
vertical	O
SPI	O
ratings	O
may	O
differ	O
for	O
a	O
given	O
scanner	O
;	O
typical	O
flatbed	O
scanners	O
use	O
a	O
horizontal	O
array	O
of	O
sensors	B-Algorithm
that	O
are	O
passed	O
across	O
the	O
bed	O
using	O
an	O
electric	O
stepper	B-Algorithm
motor	I-Algorithm
.	O
</s>
<s>
The	O
density	O
of	O
the	O
array	O
of	O
sensors	B-Algorithm
determines	O
the	O
horizontal	O
scanning	O
resolution	O
,	O
while	O
the	O
minimum	O
step	O
size	O
of	O
the	O
motor	O
determines	O
the	O
vertical	O
resolution	O
.	O
</s>
<s>
To	O
calculate	O
the	O
number	O
of	O
raw	O
data	O
bytes	O
that	O
a	O
scanned	B-Application
image	I-Application
will	O
take	O
up	O
,	O
you	O
can	O
use	O
the	O
follow	O
formula	O
:	O
</s>
