Terahertz wave spectrometry system

1. A terahertz wave spectrometry system comprising:

a terahertz wave emitter for emitting a terahertz wave to irradiate a test substance with the terahertz wave;

a light receiver that receives an absorbance of a terahertz wave transmitted through or reflected from the test substance; and

a signal processor,

wherein the signal processor, in operation,

outputs an irradiation signal to the terahertz wave emitter to irradiate the test substance with the terahertz wave while increasing or decreasing a frequency f of the terahertz wave,

acquires an intensity of the terahertz wave received by the light receiver;

acquires a function A(f) of an absorption spectrum expressing the absorbance of the terahertz wave which has been transmitted through or reflected from the test substance with respect to the frequency f, on the basis of an intensity of the terahertz wave emitted by the terahertz wave emitter and the intensity of the terahertz wave received by the light receiver;

differentiates the function A(f) with respect to the frequency f to acquire a function A′(f);

detects two or more first areas in each of which a value of the function A′(f) changes from a negative value to a positive value as the frequency f increases;

defines bottom frequencies b1, b2, . . . , bm as values of the frequency f each satisfying a formula (I) below in an m-th one of the first areas, where m is an integer of 2 or more;

function A ′( bm )=0  (I)

calculates a value of the function A(bm) at each bottom frequency bm;

forms a baseline function B(f) passing through coordinates (b1, f(b1)), coordinates (b2, f(b2)), . . . , coordinates (bm, f(bm)), or a neighborhood of these coordinates; and

subtracts the baseline function from the absorption spectrum.

2. The terahertz wave spectrometry system according to claim 1 , wherein

the signal processor, in operation,

detects two or more second areas in each of which the value of the function A′(f) changes from a positive value to a negative value as the frequency f increases;

defines peak frequencies p1, p2, . . . , pm as values of the frequency f each satisfying a formula (II) below in an m-th one of the second areas, where m is an integer of 2 or more;

function A ′( pm )=0  (II)

calculates a value of the function A(pm) at each peak frequency pm; and

excludes coordinates (bs, f(bs)) from coordinates used for forming the baseline function B(f) in a case where a formula (III) below is satisfied,

f ( ps )− f ( bs )< g   (III)

where s is an integer of 2 or more and smaller than m, and g is a predetermined value.

3. The terahertz wave spectrometry system according to claim 2 , wherein the predetermined value is 0.2.

4. The terahertz wave spectrometry system according to claim 2 , wherein

the signal processor, in operation,

excludes the coordinates (bs, f(bs)) from the coordinates used for forming the baseline function B(f) in a case where a formula (IV) below is satisfied,

| f ( bs )− f ( bs− 1)|≧ c   (IV)

where s is an integer of 2 or more and smaller than m, and c is a predetermined value.

5. The terahertz wave spectrometry system according to claim 2 , wherein

the signal processor, in operation,

excludes the coordinates (bs, f(bs)) from the coordinates used for forming the baseline function B(t) in a case where a formula (V) below is satisfied,

| f ( ps )− f ( ps− 1)|< d   (V)

where bs is larger than ps−1 and smaller than ps, s is an integer of 2 or more and smaller than m, and d is a predetermined value.

6. The terahertz wave spectrometry system according to claim 4 , wherein

the signal processor, in operation,

excludes either coordinates (bs−1, f(bs−1)) or the coordinates (bs, f(bs)) from the coordinates used for forming the baseline function B(f) in a case where the formula (IV) is not satisfied and a formula (VI) below is satisfied,

| f ( bs )− f ( bs− 1)|< e   (VI)

where s is an integer of 2 or more and smaller than m, and e is a predetermined value.

7. The terahertz wave spectrometry system according to claim 1 , wherein

the signal processor, in operation,

sets, among the coordinates corresponding to the bottom frequencies, coordinates corresponding to a minimum frequency in a measuring frequency range as a minimum frequency bottom, and coordinates corresponding to a maximum frequency in the measuring frequency range as a maximum frequency bottom; and

calculates the baseline function so that the baseline function passes through at least both the minimum frequency bottom and the maximum frequency bottom.

8. The terahertz wave spectrometry system according to claim 7 , wherein

the signal processor, in operation,

sets, among the coordinates corresponding to the bottom frequencies, coordinates located between the minimum frequency bottom and the maximum frequency bottom as an intermediate bottom; and

calculates the baseline function so that the baseline function passes through at least all of the minimum frequency bottom, the maximum frequency bottom and the intermediate bottom.

9. The terahertz wave spectrometry system according to claim wherein

the signal processor, in operation,

detect two or more second areas in each of which the value of the function A′(f) changes from a positive value to a negative value as the frequency f increases;

define peak frequencies p1, p2, . . . , pm as values of the frequency f each satisfying a formula (II) below in an m-th one of the second areas, where m is an integer of 2 or more;

function A ′( p )=0  (II)

calculate a value of the function A(pm) at each peak frequency pm;

exclude coordinates (bs, f(bs)) from coordinates used for forming the baseline function B(f) in a case where a formula (III) below is satisfied,

f ( ps )− f ( bs )< g   (III)

where ps is smaller than bs, s is an integer of 2 or more and smaller than m, and g is a predetermined value.

10. The terahertz wave spectrometry system according to claim 7 , wherein the measuring frequency range has a range of at least 1 THz.

11. The terahertz wave spectrometry system according to claim 1 , wherein the baseline function is an exponential function or a quadratic function.

12. The terahertz wave spectrometry system according to claim 1 , wherein the terahertz wave is in a frequency range from 0.1 THz to 10 THz.

13. The terahertz wave spectrometry system according to claim 1 , further comprising a display unit,

wherein

the signal processor calculates the baseline function form the absorption spectrum, and then subtracts the baseline function from the absorption spectrum to calculate a post-subtraction absorption spectrum, and

the display unit displays at least the post-subtraction absorption spectrum.

14. The terahertz wave spectrometry system according to claim 13 , wherein the display unit displays the post-subtraction absorption spectrums of two or more kinds of the test substances so that baselines of the post-subtraction absorption spectrums are overlapped.

15. The terahertz wave spectrometry system according to claim 13 , wherein the display unit displays values subtracted by the baseline function.